knuck·le 
(nŭkəl)
(nŭkəl)n.
1.
a. The prominence of the dorsal aspect of a joint of a finger, especially of one of the joints connecting the fingers to the hand.
b. A rounded protuberance formed by the bones in a joint.
2. A cut of meat centering on the carpal or tarsal joint, as of a pig.
3. The part of a hinge through which the pin passes.
4. A sharp angle formed by the meeting of two surfaces, especially two ship's timbers.
5. knuckles Brass knuckles.
tr.v. knuck·led, knuck·ling, knuck·les
Phrasal Verbs: 1. To press, rub, or hit with the knuckles.
2. To shoot (a marble) with the thumb over the bent forefinger.
knuckle down
To apply oneself earnestly to a task.
knuckle under
To yield to pressure; give in.
[Middle English knokel.]