knuckle

noun

a part of a finger at a joint where the bone is near the surface, especially where the finger joins the hand.

verb

rub or press (something, especially the eyes) with the knuckles.

Middle English knokel (originally denoting the rounded shape when a joint such as the elbow or knee is bent), from Middle Low German, Middle Dutch knökel, diminutive of knoke ‘bone’. In the mid 18th century the verb knuckle (down) expressed setting the knuckles down to shoot the taw in a game of marbles, hence the notion of applying oneself with concentration.