brass knucks vs plural

brass knucks and plural both are nouns.

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

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