stupid vs thick-skulled

stupid is a noun but thick-skulled is not a noun.

Word	Noun	Adjective	Verb	Adverb
stupid	Yes	Yes	No	No
thick-skulled	No	Yes	No	No

stupid (noun)	thick-skulled (noun)
a person who is not very bright

stupid (adjective)	thick-skulled (adjective)
lacking or marked by lack of intellectual acuity	(used informally) stupid
lacking intelligence
in a state of mental numbness especially as resulting from shock