brass knuckles vs plural form

brass knuckles and plural form both are nouns.

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

brass knuckles (noun)	plural form (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