knuckle (v.) Look up knuckle at Dictionary.com
1740, from knuckle (n.), originally in the game of marbles (putting a knuckle on the ground is the hand position preliminary to shooting). To knuckle down "apply oneself earnestly" is 1864 in American English, an extended sense from marbles; to knuckle under "submit, give in" is first recorded 1740, supposedly from the former more general sense of "knuckle" and here meaning "knee," hence "to kneel."
knuckle (n.) Look up knuckle at Dictionary.com
mid-14c., knokel "finger joint; any joint of the body, especially a knobby one; morbid lump or swelling." Perhaps in Old English, but not attested there. Common Germanic (compare Middle Low German knökel, Middle Dutch cnockel, German knöchel), literally "little bone," a diminutive of Proto-Germanic root *knuk- "bone," which is not represented in English in its simple form (but compare German Knochen "bone). For pronunciation, see kn-.