knucklehead vs stupid

knucklehead and stupid both are nouns.

Word	Noun	Adjective	Verb	Adverb
knucklehead	Yes	No	No	No
stupid	Yes	Yes	No	No

As nouns, stupid is a hypernym of knucklehead; that is, stupid is a word with a broader meaning than knucklehead:

knucklehead: a stupid person; these words are used to express a low opinion of someone's intelligence
stupid: a person who is not very bright

Other hypernyms of knucklehead include dolt, dullard, pillock, poor fish, pudden-head, pudding head, stupe, stupid person.

knucklehead (noun)	stupid (noun)
a stupid person; these words are used to express a low opinion of someone's intelligence	a person who is not very bright

knucklehead (adjective)	stupid (adjective)
	lacking or marked by lack of intellectual acuity
	lacking intelligence
	in a state of mental numbness especially as resulting from shock