1.  In human beings, the hinder part of the body, extending from the neck to the end of the spine; in other animals, that part of the body which corresponds most nearly to such part of a human being; as, the back of a horse, fish, or lobster. [1913 Webster]

2.  An extended upper part, as of a mountain or ridge. [1913 Webster]
"[The mountains] their broad bare backs upheave
Into the clouds." [1913 Webster]

3.  The outward or upper part of a thing, as opposed to the inner or lower part; as, the back of the hand, the back of the foot, the back of a hand rail. [1913 Webster]
"Methought Love pitying me, when he saw this,
Gave me your hands, the backs and palms to kiss." [1913 Webster]

4.  The part opposed to the front; the hinder or rear part of a thing; as, the back of a book; the back of an army; the back of a chimney. [1913 Webster]

5.  The part opposite to, or most remote from, that which fronts the speaker or actor; or the part out of sight, or not generally seen; as, the back of an island, of a hill, or of a village. [1913 Webster]

6.  The part of a cutting tool on the opposite side from its edge; as, the back of a knife, or of a saw. [1913 Webster]

7.  A support or resource in reserve. [1913 Webster]
"This project
Should have a back or second, that might hold,
If this should blast in proof." [1913 Webster]

8.  The keel and keelson of a ship. [1913 Webster]

9.  The upper part of a lode, or the roof of a horizontal underground passage. [1913 Webster]

10.  A garment for the back; hence, clothing. [1913 Webster]
"A bak to walken inne by daylight." [1913 Webster]

1.  Being at the back or in the rear; distant; remote; as, the back door; back settlements. [1913 Webster]

2.  Being in arrear; overdue; as, back rent. [1913 Webster]

3.  Moving or operating backward; as, back action. [1913 Webster]

1.  To get upon the back of; to mount. [1913 Webster]
"I will back him [a horse] straight." [1913 Webster]

2.  To place or seat upon the back. [1913 Webster]
"Great Jupiter, upon his eagle backed,
Appeared to me." [1913 Webster]

3.  To drive or force backward; to cause to retreat or recede; as, to back oxen. [1913 Webster]

4.  To make a back for; to furnish with a back; as, to back books. [1913 Webster]

5.  To adjoin behind; to be at the back of. [1913 Webster]
"A garden . . . with a vineyard backed." [1913 Webster]
"The chalk cliffs which back the beach." [1913 Webster]

6.  To write upon the back of; as, to back a letter; to indorse; as, to back a note or legal document. [1913 Webster]

7.  To support; to maintain; to second or strengthen by aid or influence; as, to back a friend. Macaulay. [1913 Webster]
"Have still found it necessary to back and fortify their laws with rewards and punishments." [1913 Webster]
"The mate backed the captain manfully." [1913 Webster]

8.  To bet on the success of; -- as, to back a race horse. [1913 Webster]

1.  To move or go backward; as, the horse refuses to back. [1913 Webster]

2.  To change from one quarter to another by a course opposite to that of the sun; -- used of the wind. [1913 Webster]

3.  To stand still behind another dog which has pointed; -- said of a dog. [1913 Webster]
"Cleon at first . . . was willing to go; but, finding that he [Nicias] was in earnest, he tried to back out." [1913 Webster]

1.  In, to, or toward, the rear; as, to stand back; to step back. [1913 Webster]

2.  To the place from which one came; to the place or person from which something is taken or derived; as, to go back for something left behind; to go back to one's native place; to put a book back after reading it. [1913 Webster]

3.  To a former state, condition, or station; as, to go back to private life; to go back to barbarism. [1913 Webster]

4.  (Of time) In times past; ago. Gladstone. [1913 Webster]

5.  Away from contact; by reverse movement. [1913 Webster]
"The angel of the Lord . . . came, and rolled back the stone from the door." [1913 Webster]

6.  In concealment or reserve; in one's own possession; as, to keep back the truth; to keep back part of the money due to another. [1913 Webster]

7.  In a state of restraint or hindrance. [1913 Webster]
"The Lord hath kept thee back from honor." [1913 Webster]

8.  In return, repayment, or requital. [1913 Webster]
"What have I to give you back?" [1913 Webster]

9.  In withdrawal from a statement, promise, or undertaking; as, he took back the offensive words. [1913 Webster]

10.  In arrear; as, to be back in one's rent. [1913 Webster]

back, n., adv., v., & adj.
--n.
1 a the rear surface of the human body from the shoulders to the hips. b the corresponding upper surface of an animal's body. c the spine (fell and broke his back). d the keel of a ship.
2 a any surface regarded as corresponding to the human back, e.g. of the head or hand, or of a chair. b the part of a garment that covers the back.
3 a the less active or visible or important part of something functional, e.g. of a knife or a piece of paper (write it on the back). b the side or part normally away from the spectator or the direction of motion or attention, e.g. of a car, house, or room (stood at the back).
4 a a defensive player in field games. b this position.
5 (the Backs) the grounds of Cambridge colleges which back on to the River Cam.
--adv.
1 to the rear; away from what is considered to be the front (go back a bit; ran off without looking back).
2 a in or into an earlier or normal position or condition (came back late; went back home; ran back to the car; put it back on the shelf). b in return (pay back).
3 in or into the past (back in June; three years back).
4 at a distance (stand back from the road).
5 in check (hold him back).
6 (foll. by of) US behind (was back of the house).
--v.
1 tr. a help with moral or financial support. b bet on the success of (a horse etc.).
2 tr. & intr. move, or cause (a vehicle etc.) to move, backwards.
3 tr. a put or serve as a back, background, or support to. b Mus. accompany.
4 tr. lie at the back of (a beach backed by steep cliffs).
5 intr. (of the wind) move round in an anticlockwise direction.
--adj.
1 situated behind, esp. as remote or subsidiary (backstreet; back teeth).
2 of or relating to the past; not current (back pay; back issue).
3 reversed (back flow).

Idiom:
at a person's back in pursuit or support. at the back of one's mind remembered but not consciously thought of. back and forth to and fro. back bench a back-bencher's seat in the House of Commons. back-bencher a member of Parliament not holding a senior office. back-boiler Brit. a boiler behind and integral with a domestic fire. back-breaking (esp. of manual work) extremely hard. back country esp. Austral. & NZ an area away from settled districts. back-crawl = BACKSTROKE. back-cross Biol.
1 cross a hybrid with one of its parents.
2 an instance or the product of this. back door a secret or ingenious means of gaining an objective. back-door adj. (of an activity) clandestine, underhand (back-door deal). back down withdraw one's claim or point of view etc.; concede defeat in an argument etc. back-down n. an instance of backing down. back-fill refill an excavated hole with the material dug out of it.
back-formation
1 the formation of a word from its seeming derivative (e.g. laze from lazy).
2 a word formed in this way.
back number
1 an issue of a periodical earlier than the current one.
2 sl. an out-of-date person or thing. the back of beyond a very remote or inaccessible place.
back off
1 draw back, retreat.
2 abandon one's intention, stand, etc. back on to have its back adjacent to (the house backs on to a field). back out (often foll. by of) withdraw from a commitment. back passage colloq. the rectum.
back-pedal (-pedalled, -pedalling; US -pedaled, -pedaling)
1 pedal backwards on a bicycle etc.
2 reverse one's previous action or opinion. back-projection the projection of a picture from behind a translucent screen for viewing or filming. back room (often (with hyphen) attrib.) a place where secret work is done. back-scattering the scattering of radiation in a reverse direction. back seat an inferior position or status. back-seat driver a person who is eager to advise without responsibility (orig. of a passenger in a car etc.). back slang slang using words spelt backwards (e.g. yob). back-stop = LONGSTOP. back talk US = BACKCHAT. back to back with backs adjacent and opposite each other (we stood back to back). back-to-back adj. esp. Brit. (of houses) with a party wall at the rear.
back to front
1 with the back at the front and the front at the back.
2 in disorder. back-to-nature (usu. attrib.) applied to a movement or enthusiast for the reversion to a simpler way of life.
back up
1 give (esp. moral) support to.
2 Computing make a spare copy of (data, a disk, etc.).
3 (of running water) accumulate behind an obstruction.
4 reverse (a vehicle) into a desired position.
5 US form a queue of vehicles etc., esp. in congested traffic. back water reverse a boat's forward motion using oars. get (or put) a person's back up annoy or anger a person. get off a person's back stop troubling a person. go back on fail to honour (a promise or commitment). know like the back of one's hand be entirely familiar with. on one's back injured or ill in bed. on the back burner see BURNER. put one's back into approach (a task etc.) with vigour. see the back of see SEE(1).
turn one's back on
1 abandon.
2 ignore. with one's back to (or up against) the wall in a desperate situation; hard-pressed.

Derivative:
backer n. (in sense 1 of v.). backless adj.

Etymology:
OE b{aelig}c f. Gmc

VB  aid, assist, help, succor, lend one's aid, come to the aid, of, contribute, subscribe to, bring aid, give aid, furnish aid, afford aid, supply aid, give a helping hand, stretch a hand, lend a helping hand, lend a hand, bear a helping hand, hold out a hand, hold out a helping hand, give one a life, give one a cast, give one a turn, take by the hand, take in tow, help a lame dog over a stile, lend wings to, relieve, rescue, set up, set agoing, set on one's legs, bear through, pull through, give new life to, be the making of, reinforce, reenforce, recruit, set forward, put forward, push forward, give a lift, give a shove, give an impulse to, promote, further, forward, advance expedite, speed, quicken, hasten, support, sustain, uphold, prop, hold up, bolster, cradle, nourish, nurture, nurse, dry nurse, suckle, put out to nurse, manure, cultivate, force, foster, cherish, foment, feed the flame, fan the flame, serve, do service to, tender to, pander to, administer to, subminister to, minister to, tend, attend, wait on, take care of, entertain, smooth the bed of death, oblige, accommodate, consult the wishes of, humor, cheer, encourage, second, stand by, back, back up, pay the piper, abet, work for, make interest for, stick up for, take up the cudgels for, take up the cause of, espouse the cause of, adopt the cause of, advocate, beat up for recruits, press into the service, squire, give moral support to, keep in countenance, countenance, patronize, lend oneself to, lend one's countenance to, smile upon, shine upon, favor, befriend, take in hand, enlist under the banners of, side with, be of use to, subserve, benefit, render a service, conduce.

VB  amount to, come to, mount up to, touch the pocket, draw, draw upon, indorse, issue, utter, discount, back, demonetize, remonetize, fiscalize, monetize, circulate, be in circulation, be out of circulation, mint (coins), coin, print (paper currency), inflate, deflate, debase, devalue, revalue, circulate, put in circulation, withdraw from circulation, exchange currencies, change money, charge interest, pay interest, lose interest.

N  rear, back, posteriority, rear rank, rear guard, background, hinterland, occiput, nape, chine, heels, tail, rump, croup, buttock, posteriors, backside scut, breech, dorsum, loin, dorsal region, lumbar region, hind quarters, aitchbone, natch, natch bone, stern, poop, afterpart, heelpiece, crupper, wake, train, reverse, other side of the shield, back, rear, hind, hinder, hindmost, hindermost, postern, posterior, dorsal, after, caudal, lumbar, mizzen, tergal, behind, in the rear, in the background, behind one's back, at the heels of, at the tail of, at the back of, back to back, after, aft, abaft, astern, sternmost, aback, rearward, ogni medaglia ha il suo rovescio, the other side of the coin, rear, back, posteriority, rear rank, rear guard, background, hinterland, occiput, nape, chine, heels, tail, rump, croup, buttock, posteriors, backside scut, breech, dorsum, loin, dorsal region, lumbar region, hind quarters, aitchbone, natch, natch bone, stern, poop, afterpart, heelpiece, crupper, wake, train, reverse, other side of the shield, back, rear, hind, hinder, hindmost, hindermost, postern, posterior, dorsal, after, caudal, lumbar, mizzen, tergal, behind, in the rear, in the background, behind one's back, at the heels of, at the tail of, at the back of, back to back, after, aft, abaft, astern, sternmost, aback, rearward, ogni medaglia ha il suo rovescio, the other side of the coin.

N  convexity, prominence, projection, swelling, gibbosity, bilge, bulge, protuberance, protrusion, camber, cahot, thank-ye-ma'am, swell, intumescence, tumour, tumor, tubercle, tuberosity, excrescence, hump, hunch, bunch, boss, embossment, hub, hubble, tooth, knob, elbow, process, apophysis, condyle, bulb, node, nodule, nodosity, tongue, dorsum, bump, clump, sugar loaf, bow, mamelon, molar, belly, corporation, pot belly, gut, withers, back, shoulder, lip, flange, pimple, zit, wen, wheel, papula, pustule, pock, proud flesh, growth, sarcoma, caruncle, corn, wart, pappiloma, furuncle, polypus, fungus, fungosity, exostosis, bleb, blister, blain, boil, airbubble, blob, papule, verruca, papilla, nipple, teat, tit, titty, boob, knocker, pap, breast, dug, mammilla, proboscis, nose, neb, beak, snout, nozzle, schnoz, peg, button, stud, ridge, rib, jutty, trunnion, snag, cupola, dome, arch, balcony, eaves, pilaster, relief, relievo, cameo, bassorilievo, mezzorilevo, altorivievo, low relief, bas relief, high relief, hill, cape, promontory, mull, forehead, foreland, point of land, mole, jetty, hummock, ledge, spur, naze, ness, convex, prominent, protuberant, projecting, bossed, embossed, bossy, nodular, bunchy, clavate, clavated, claviform, hummocky, moutonne, mammiliform, papulous, papilose, hemispheric, bulbous, bowed, arched, bold, bellied, tuberous, tuberculous, tumous, cornute, odontoid, lentiform, lenticular, gibbous, club shaped, hubby, hubbly, knobby, papillose, saddle-shaped, selliform, subclavate, torose, ventricose, verrucose, salient, in relief, raised, repousse, bloated, (expanded).

N  regress, regression, retrocession, retrogression, retrograduation, retroaction, reculade, retreat, withdrawal, retirement, remigration, recession, recess, crab-like motion, refluence, reflux, backwater, regurgitation, ebb, return, resilience reflection, reflexion (recoil), flip-flop, volte-face, counter motion, retrograde motion, backward movement, motion in reverse, counter movement, counter march, veering, tergiversation, recidivation, backsliding, fall, deterioration, recidivism, recidivity, reversal, relapse, turning point, receding, retrograde, retrogressive, regressive, refluent, reflex, recidivous, resilient, crab-like, balky, reactionary, back, backwards, reflexively, to the right about, a reculons, a rebours, revenons a nos moutons, as you were.

answer back, back and fill, back and forth, back away, back brace, back breaker, back burner, back channel, back circle, back country, back door, back down, back end, back entrance, back exercise, back fire, back judge, back matter, back number, back of beyond, back off, back out, back pack, back porch, back room, back saw, back scratching, back seat, back stairs, back street, back talk, back to back, back tooth, back up, back way, beat back, bounce back, break one's back, bring back, buy back, call back, carry back, ceding back, choke back, come back, come back at, coming back, cut back, date back, die back, double back, draw back, drive back, drop back, fall back, fat back, feed back, fight back, flanker back, flash back, force back, from way back, get back, get back at, give back, go back, go back on, go back over, hang back, hark back, hold back, keep back, kick back, knock back, lean back, look back, move back, move back and forth, pat on the back, pay back, paying back, play back, pull back, push back, put back, rear back, relation back, roll back, running back, send back, set back, shrink back, sink back, sit back, slide back, slip back, snap back, stand back, strike back, take back, talk back, think back, throw back, thrust back, toss back, travel back and forth, trim back, turn back, water back, watteau back, win back