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mid-14c., knokel "finger joint; any joint of the body, especially a knobby one; morbid lump or swelling;" common Germanic (cf. Middle Low German knökel, Middle Dutch cnockel, German knöchel), literally "little bone," a diminutive of Proto-Germanic root *knuck- "bone" (cf. German Knochen "bone).
As a verb from 1740, originally in the game of marbles. To knuckle down "apply oneself earnestly" is 1864 in American English, extended from marbles (putting a knuckle on the ground in assuming the hand position preliminary to shooting); 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." The face-busting knuckle-duster is from 1858 (a duster was a type of protective coat worn by workmen).
knuckle knuck·le (nŭk'əl)
The prominence of the dorsal aspect of a joint of a finger, especially of one of the joints that connect the fingers to the hand.
A rounded protuberance formed by the bones in a joint.
A kink or loop of intestine, as in a hernia.
To work hard and seriously; stop loafing; buckle down
[1866+; fr the act of putting one's knuckles down to the taw or marble preparing for a careful shot in the game of marbles, a use dating fr the mid-18th century]