brass knuckles vs knuckles

brass knuckles and knuckles both are nouns.

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

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

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

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

brass knuckles (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