brass knuckles vs plural

brass knuckles and plural both are nouns.

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

brass knuckles (noun)	plural (noun)
(used in the plural) a small metal weapon; worn over the knuckles on the back of the hand	the form of a word that is used to denote more than one

brass knuckles (adjective)	plural (adjective)
	grammatical number category referring to two or more items or units
	composed of more than one member, set, or kind