1.  In man, the joint in the middle part of the leg. [1913 Webster]

2.  The joint, or region of the joint, between the thigh and leg. [1913 Webster]

3.  A piece of timber or metal formed with an angle somewhat in the shape of the human knee when bent. [1913 Webster]

4.  A bending of the knee, as in respect or courtesy. [1913 Webster]
"Give them title, knee, and approbation." [1913 Webster]

   To supplicate by kneeling. [1913 Webster]
"Fall down, and knee
The way into his mercy." [1913 Webster]

knee, n. & v.
--n.
1 a (often attrib.) the joint between the thigh and the lower leg in humans. b the corresponding joint in other animals. c the area around this. d the upper surface of the thigh of a sitting person; the lap (held her on his knee).
2 the part of a garment covering the knee.
3 anything resembling a knee in shape or position, esp. a piece of wood or iron bent at an angle, a sharp turn in a graph, etc.
--v.tr. (knees, kneed, kneeing)
1 touch or strike with the knee (kneed the ball past him; kneed him in the groin).
2 colloq. cause (trousers) to bulge at the knee.

Idiom:
bend (or bow) the knee kneel, esp. in submission. bring a person to his or her knees reduce a person to submission. knee-bend the action of bending the knee, esp. as a physical exercise in which the body is raised and lowered without the use of the hands. knee-breeches close-fitting trousers reaching to or just below the knee. knee-deep
1 (usu. foll. by in) a immersed up to the knees. b deeply involved.
2 so deep as to reach the knees. knee-high so high as to reach the knees. knee-hole a space for the knees, esp. under a desk.
knee-jerk
1 a sudden involuntary kick caused by a blow on the tendon just below the knee.
2 (attrib.) predictable, automatic, stereotyped.
knee-joint
1 = senses 1a, b of n.
2 a joint made of two pieces hinged together. knee-length reaching the knees. knee-pan the kneecap. knees-up Brit. colloq. a lively party or gathering. on (or on one's) bended knee (or knees) kneeling, esp. in supplication, submission, or worship.

Etymology:
OE cneo(w)

N  angularity, angularness, aduncity, angle, cusp, bend, fold, notch, fork, bifurcation, elbow, knee, knuckle, ankle, groin, crotch, crutch, crane, fluke, scythe, sickle, zigzag, kimbo, akimbo, corner, nook, recess, niche, oriel, coign, right angle, obliquity, angle of 45 degrees, miter, acute angle, obtuse angle, salient angle, reentering angle, spherical angle, angular measurement, angular elevation, angular distance, angular velocity, trigonometry, goniometry, altimetry, clinometer, graphometer, goniometer, theodolite, sextant, quadrant, dichotomy, triangle, trigon, wedge, rectangle, square, lozenge, diamond, rhomb, rhombus, quadrangle, quadrilateral, parallelogram, quadrature, polygon, pentagon, hexagon, heptagon, octagon, oxygon, decagon, pyramid, cone, Platonic bodies, cube, rhomboid, tetrahedron, pentahedron, hexahedron, octahedron, dodecahedron, icosahedron, eicosahedron, prism, pyramid, parallelopiped, curb roof, gambrel roof, mansard roof, angular, bent, crooked, aduncous, uncinated, aquiline, jagged, serrated, falciform, falcated, furcated, forked, bifurcate, zigzag, furcular, hooked, dovetailed, knock kneed, crinkled, akimbo, kimbo, geniculated, oblique, fusiform, wedge-shaped, cuneiform, cuneate, multangular, oxygonal, triangular, trigonal, trilateral, quadrangular, quadrilateral, foursquare, rectangular, square, multilateral, polygonal, cubical, rhomboid, rhomboidal, pyramidal.

housemaid's knee, human knee, knee bend, knee brace, knee breeches, knee deep, knee high, knee jerk, knee jerk liberal, knee jerk reaction, knee joint, knee pad, knee pants, knee piece, water on the knee