Definition of: knuckle

(nuk′əl) noun
1. One of the joints of the fingers, or the region about it; especially, one of the joints connecting the fingers to the rest of the hand.
2. The knee or ankle joint of certain animals, used as foods.
3. The central tubular projection of a hinge through which the pin passes.
4. In shipbuilding, an angular fitting of timbers.
5. plural U.S. A children's game of marbles.
—brass knuckles A device of metal, fitting over the knuckles, used as a protection for them in striking and to add force to the blow; also knuck′le–dust′er.
—v.i. knuck·led, knuck·ling To hold the knuckles on the ground in shooting a marble.
—to knuckle down
1. To apply oneself seriously and assiduously.
2. To yield; submit; give in: also knuckle under. [ME knokel <LG. Cf. G knöchel, dim. of knochen bone.]