1.  A measure of length, containing six feet; the space to which a man can extend his arms; -- used chiefly in measuring cables, cordage, and the depth of navigable water by soundings. [1913 Webster]

2.  The measure or extant of one's capacity; depth, as of intellect; profundity; reach; penetration. [1913 Webster]
"Another of his fathom they have none
To lead their business." [1913 Webster]

1.  To encompass with the arms extended or encircling; to measure by throwing the arms about; to span. Purchas. [1913 Webster]

2.  To measure by a sounding line; especially, to sound the depth of; to penetrate, measure, and comprehend; to get to the bottom of. Dryden. [1913 Webster]
"The page of life that was spread out before me seemed dull and commonplace, only because I had not fathomed its deeper import." [1913 Webster]

fathom, n. & v.
--n. (pl. often fathom when prec. by a number)
1 a measure of six feet, esp. used in taking depth soundings.
2 Brit. a quantity of wood six feet square in cross-section.
--v.tr.
1 grasp or comprehend (a problem or difficulty).
2 measure the depth of (water) with a sounding-line.

Derivative:
fathomable adj. fathomless adj.

Etymology:
OE f{aelig}thm outstretched arms f. Gmc

N  length, longitude, span, mileage, distance, line, bar, rule, stripe, streak, spoke, radius, lengthening, prolongation, production, protraction, tension, tensure, extension, line, nail, inch, hand, palm, foot, cubit, yard, ell, fathom, rood, pole, furlong, mile, league, chain, link, arpent, handbreadth, jornada, kos, vara, astronomical unit, AU, light- year, parsec, nanometer, nm, micron, micrometer, millimicron, millimeter, mm, centimeter, cm, meter, kilometer, km, pedometer, perambulator, scale, long, longsome, lengthy, wiredrawn, outstretched, lengthened, sesquipedalian, interminable, no end of, macrocolous, linear, lineal, longitudinal, oblong, as long as my arm, as long as today and tomorrow, unshortened &c (shorten), lengthwise, at length, longitudinally, endlong, along, tandem, in a line, in perspective, from end to end, from stem to stern, from head to foot, from the crown of the head to the sole of the foot, from top to toe, fore and aft.

VB  be deep, render deep, deepen, plunge, sound, fathom, plumb, cast the lead, heave the lead, take soundings, make soundings, dig.

VB  make inquiry, inquire, ask, seek, search, look for, look about for, look out for, scan, reconnoiter, explore, sound, rummage, ransack, pry, peer, look round, look over, go over, look through, go through, spy, overhaul, ask about, inquire about, scratch the head, slap the forehead, look into every hole and corner, peer into every hole and corner, pry into every hole and corner, nose, trace up, search out, hunt down, hunt out, fish out, ferret out, unearth, leave no stone unturned, seek a clue, seek a clew, hunt, track, trail, mouse, dodge, trace, follow the trail, follow the scent, pursue, beat up one's quarters, fish for, feel for, investigate, take up an inquiry, institute an inquiry, pursue an inquiry, follow up an inquiry, conduct an inquiry, carry on an inquiry, carry out an inquiry, prosecute an inquiry, look at, look into, preexamine, discuss, canvass, agitate, examine, study, consider, calculate, dip into, dive into, delve into, go deep into, make sure of, probe, sound, fathom, probe to the bottom, probe to the quick, scrutinize, analyze, anatomize, dissect, parse, resolve, sift, winnow, view in all its phases, try in all its phases, thresh out, bring in question, bring into question, subject to examination, put to the proof, audit, tax, pass in review, take into consideration, take counsel, question, demand, put the question, pop the question, propose the question, propound the question, moot the question, start the question, raise the question, stir the question, suggest the question, put forth the question, ventilate the question, grapple with the question, go into a question, question, put to the question, interrogate, pump, subject to interrogation, subject to examination, cross-question, cross-examine, press for an answer, give the third degree, put to the inquisition, dodge, catechize, require an answer, pick the brains of, suck the brains of, feel the pulse, get the lay of the land, see how the wind is blowing, put one's ear to the ground, be in question, undergo examination.

VB  answer, respond, reply, rebut, retort, rejoin, give for answer, return for answer, acknowledge, echo, explain, solve, discover, fathom, hunt out, satisfy, set at rest, determine.

VB  measure, mete, determine, assay, evaluate, value, assess, rate, appraise, estimate, form an estimate, set a value on, appreciate, standardize, span, pace step, apply the compass, gauge, plumb, probe, sound, fathom, heave the log, heave the lead, survey, weigh, take an average, graduate.

VB  discover, find, determine, evolve, learn, fix upon, pick up, find out, trace out, make out, hunt out, fish out, worm out, ferret out, root out, fathom, bring out, draw out, educe, elicit, bring to light, dig out, grub up, fish up, unearth, disinter, solve, resolve, elucidate, unriddle, unravel, unlock, crack, crack open, pick up, open the lock, find a clue, find clew a to, find the key to the riddle, interpret, disclose, trace, get at, hit it, have it, lay one's finger, lay one's hands upon, spot, get at the truth, arrive at the truth, put the saddle on the right horse, hit the right nail on the head, be near the truth, be warm, get warmer, burn, smoke, scent, sniff, catch a whiff of, smell a rat, open the eyes to, see through, see daylight, see in its true colors, see the cloven foot, detect, catch, catch tripping, pitch upon, fall upon, light upon, hit upon, stumble upon, pop upon, come across, come onto, meet with, meet up with, fall in with, recognize, realize, verify, make certain of, identify, Int, eureka!, aha!, I've got it!.

VB  know, ken, scan, wot, wot aware, be aware of, ween, weet, trow, have, possess, conceive, apprehend, comprehend, take, realize, understand, savvy, appreciate, fathom, make out, recognize, discern, perceive, see, get a sight-of, experience, know full well, have some knowledge of, possess some knowledge of, be au courant, have in one's head, have at one' fingers ends, know by heart, know by rote, be master of, connaitre le dessous des cartes, know what's what, see one's way, discover, come to one's knowledge.