'knuckle' originates from Old English, specifically the word 'cnucel', where the root meant 'a small protuberance, knob' (relating to a finger joint).

'knuckle' changed from Old English 'cnucel' through Middle English forms (such as 'knokel'/'knuckel') and eventually became the modern English 'knuckle'. The verb sense (to hit or to bend the knuckles) developed from the noun.

Initially, it meant 'a small rounded protuberance or knob (including a finger joint)', but over time it became the specific anatomical term for the joint of a finger and then extended to verb senses like 'to strike with the knuckles' and figuratively 'to yield'.