1911 Encyclopædia Britannica/Knuckle

KNUCKLE (apparently the diminutive of a word for “bone,” found in Ger. Knochen), the joint of a finger, which, when the hand is shut, is brought into prominence. In mechanical use the word is applied to the round projecting part of a hinge through which the pin is run, and in ship-building to an acute angle on some of the timbers. A “knuckle-duster,” said to have originally come from the criminal slang of the United States, is a brass or metal instrument fitting on to the hand across the knuckles, with projecting studs and used for inflicting a brutal blow.