stupid vs knucklehead

stupid and knucklehead both are nouns.

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

As nouns, knucklehead is a hyponym of stupid; that is, knucklehead is a word with a more specific, narrower meaning than stupid:

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

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

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