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.
      [1913 Webster]

   3. The joint of a plant. [Obs.] --Bacon.
      [1913 Webster]

   4. (Mech.) The joining parts of a hinge through which the pin
      or rivet passes; a knuckle joint.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

knuckles \knuc"kles\ n. pl.
   a small metal weapon, worn over the knuckles on the back of
   the hand; called also brass knuckles and knuckle duster.

   Syn: brass knucks, knucks, brass knuckles, knuckle duster.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

knuckle duster
    n 1: a small metal weapon; worn over the knuckles on the back of
         the hand [syn: brass knucks, knucks, brass knuckles,
         knuckles, knuckle duster]