The Collaborative International Dictionary of English v.0.48:

Knuckle \Knuc"kle\, n. [OE. knokel, knokil, AS. cuncel; akin to
   D. knokkel, OFries. knokele, knokle, G. kn["o]chel, Sw.
   knoge, Dan. knokkel, G. knochen bone, and perh. to E. knock.]
   1. The joint of a finger, particularly when made prominent by
      the closing of the fingers. --Davenant.
      [1913 Webster]

   2. The kneejoint, or middle joint, of either leg of a
      quadruped, especially of a calf; -- formerly used of the
      kneejoint of a human being.
      [1913 Webster]

            With weary knuckles on thy brim she kneeled sadly
            down.                                 --Golding.
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   3. The joint of a plant. [Obs.] --Bacon.
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   4. (Mech.) The joining parts of a hinge through which the pin
      or rivet passes; a knuckle joint.
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   5. (Shipbuilding) A convex portion of a vessel's figure where
      a sudden change of shape occurs, as in a canal boat, where
      a nearly vertical side joins a nearly flat bottom.
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   6. A contrivance, usually of brass or iron, and furnished
      with points, worn to protect the hand, to add force to a
      blow, and to disfigure the person struck; -- called also
      knuckle duster, knuckles or brass knuckles. [Slang.]
      [1913 Webster]

   Knuckle joint (Mach.), a hinge joint, in which a projection
      with an eye, on one piece, enters a jaw between two
      corresponding projections with eyes, on another piece, and
      is retained by a pin which passes through the eyes and
      forms the pivot.

   Knuckle of veal (Cookery), the lower part of a leg of veal,
      from the line of the body to the knuckle.
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The Collaborative International Dictionary of English v.0.48:

Knuckle \Knuc"kle\, v. t.
   To beat with the knuckles; to pummel. [R.] --Horace Smith.
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The Collaborative International Dictionary of English v.0.48:

Knuckle \Knuc"kle\, v. i. [imp. & p. p. Knuckled;; p. pr. &
   vb. n. Knuckling.]
   To yield; to submit; -- used with down, to, or under.
   [1913 Webster]

   To knuckle to.
   (a) To submit to in a contest; to yield to. [Colloq.] See To
       knock under, under Knock, v. i.
   (b) To apply one's self vigorously or earnestly to; as, to
       knuckle to work. [Colloq.]
       [1913 Webster]

WordNet (r) 3.0 (2006):

knuckle
    n 1: a joint of a finger when the fist is closed [syn:
         knuckle, knuckle joint, metacarpophalangeal joint]
    v 1: press or rub with the knuckles
    2: shoot a marble while keeping one's knuckles on the ground

Moby Thesaurus II by Grady Ward, 1.0:

78 Moby Thesaurus words for "knuckle":
   Chateaubriand, ankle, articulation, blade roast, boundary, bow,
   breast, brisket, butt, capitulate, cave, cervix, chuck,
   chuck roast, clinch, clod, closure, cold cuts, connecting link,
   connecting rod, connection, coupling, defer, dovetail, elbow,
   embrace, filet mignon, flank, gliding joint, hinge, hinged joint,
   hip, interface, join, joining, joint, juncture, knee,
   knuckle under, link, loin, miter, mortise, neck, pivot,
   pivot joint, plate, plate piece, pot roast, rabbet, rack,
   rib roast, ribs, roast, rolled roast, round, rump, rump roast,
   saddle, scarf, seam, shank, short ribs, shoulder, shoulder clod,
   sirloin, stitch, submit, succumb, suture, symphysis, tenderloin,
   tie rod, toggle, toggle joint, union, weld, wrist