knuckle

Concise Oxford English Dictionary © 2008 Oxford University Press:
knuckle/ˈnʌkl/

▶noun

• 1 a part of a finger at a joint where the bone is near the surface.

• 2 a projection of the carpal or tarsal joint of a quadruped.

  ■ a joint of meat consisting of such a projection together with the adjoining parts.

▶verb

• 1 (knuckle down) apply oneself seriously to a task.

  ■ (also knuckle under) submit.

• 2 rub or press with the knuckles.

– phrases
near the knuckle Brit. informal verging on the indecent or offensive.
rap someone on (or over) the knuckles rebuke or criticize someone.

– derivatives
knuckly adjective.

– origin ME knokel, from Mid. Low Ger., MDu. knökel, dimin. of knoke ‘bone’; knuckle down derives from the custom of placing one's hands with the knuckles on the ground at the start of a game of marbles.