brass knucks vs knuckles

brass knucks and knuckles both are nouns.

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

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

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

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

brass knucks (noun)	knuckles (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