Knuckle n. : The joint of a finger, particularly when made prominent by the closing of the fingers.

Knuckle n. : The kneejoint, or middle joint, of either leg of a quadruped, especially of a calf; -- formerly used of the kneejoint of a human being.

Knuckle n. : The joint of a plant.

Knuckle n. : The joining pars of a hinge through which the pin or rivet passes; a knuckle joint.

Knuckle n. : 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.

Knuckle n. : 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.

Knuckle v. i. : To yield; to submit; -- used with down, to, or under.

Knuckle v. t. : To beat with the knuckles; to pommel.

Example:
What is the difference between Knuckle ball and Knuckle curve ball?

Taxonomy
knuckles, free encyclopedia, finger, Knuckle cracking, search, Unsourced, prominence, diminutive, bone, Knochen, Knöchlein, Medically, fingers and toes, Knuckle sandwich, Personal tools, Interaction, Toolbox, Languages.

Defination from the web:
Knuckle (disambiguation). The knuckles are the joints of the fingers and toes, which are brought ... The cause of the popping or cracking noise heard when "cracking your knuckles" is still debated, but it is thought to be caused by cavitation within the joint. ...