Source : Webster's Revised Unabridged Dictionary (1913)

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

   {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.]

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.

   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.

            With weary knuckles on thy brim she kneeled sadly
            down.                                 --Golding.

   3. The joint of a plant. [Obs.] --Bacon.

   4. (Mech.) The joining pars of a hinge through which the pin
      or rivet passes; a knuckle joint.

   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.

   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; as, brass
      knuckles; -- called also {knuckle duster}. [Slang.]

   {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.

Knuckle \Knuc"kle\, v. t.
   To beat with the knuckles; to pommel. [R.] --Horace Smith.

Source : WordNet®

knuckle
     n : a joint of a finger when the fist is closed [syn: {knuckle
         joint}, {metacarpophalangeal joint}]

knuckle
     v 1: press or rub with the knuckles
     2: shoot a marble while keeping one's knuckles on the ground