a joint of a finger, esp. one of the articulations of a metacarpal with a phalanx.

the rounded prominence of such a joint when the finger is bent.

a joint of meat, consisting of the parts about the carpal or tarsal joint of a quadruped.

an angle or protrusion at the intersection of two members or surfaces, as in the timbers of a ship or in a roof.

brass knuckles.

a cylindrical projecting part on a hinge, through which an axis or pin passes; the joint of a hinge.

(in a wire mesh) a bend in a wire crossing another wire.

(on a chair arm) one of the ridges left at the front end by longitudinal flutes carved to accommodate the fingers.

Nautical . a pronounced edge formed by a change in the form of the shell of a hull.

to rub or press with the knuckles.

Marbles . to shoot (a marble) from the thumb and forefinger. ?

knuckle down,

to apply oneself vigorously and earnestly; become serious

Just knuckle down for an hour or so and finish the work.

Also, knuckle under. to submit; yield.