Knuckle  —n. 1 bone at a finger-joint, esp. That connecting the finger to the hand. 2 a knee- or ankle-joint of a quadruped. B this as a joint of meat, esp. Of bacon or pork. —v. (-ling) strike, press, or rub with the knuckles.  knuckle down (often foll. By to) 1 apply oneself seriously (to a task etc.). 2 (also knuckle under) give in; submit. [low german or dutch diminutive of knoke bone]