stupid vs soft-witted

stupid is a noun but soft-witted is not a noun.

Word	Noun	Adjective	Verb	Adverb
stupid	Yes	Yes	No	No
soft-witted	No	Yes	No	No

stupid (noun)	soft-witted (noun)
a person who is not very bright

stupid (adjective)	soft-witted (adjective)
lacking or marked by lack of intellectual acuity	(of especially persons) lacking sense or understanding or judgment
lacking intelligence
in a state of mental numbness especially as resulting from shock