brass knuckles vs weapon

brass knuckles and weapon both are nouns.

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

As nouns, weapon is a hypernym of brass knuckles; that is, weapon is a word with a broader meaning than brass knuckles:

brass knuckles: (used in the plural) a small metal weapon; worn over the knuckles on the back of the hand
weapon: any instrument or instrumentality used in fighting or hunting

Other hypernyms of brass knuckles include arm, weapon system.

brass knuckles (noun)	weapon (noun)
(used in the plural) a small metal weapon; worn over the knuckles on the back of the hand	any instrument or instrumentality used in fighting or hunting
	a means of persuading or arguing