brass knuckles vs brass knucks

brass knuckles and brass knucks both are nouns.

Word	Noun	Adjective	Verb	Adverb
brass knuckles	Yes	No	No	No
brass knucks	Yes	No	No	No

As nouns, brass knuckles and brass knucks are synonyms defined as:

brass knuckles and brass knucks: (used in the plural) a small metal weapon; worn over the knuckles on the back of the hand

Other synonyms of brass knuckles include knuckle duster, knuckles, knucks.

brass knuckles (noun)	brass knucks (noun)
(used in the plural) a small metal weapon; worn over the knuckles on the back of the hand	(used in the plural) a small metal weapon; worn over the knuckles on the back of the hand