stupid vs witless

stupid is a noun but witless is not a noun.

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

stupid (noun)	witless (noun)
a person who is not very bright

stupid (adjective)	witless (adjective)
lacking or marked by lack of intellectual acuity	(used as complement) to the utmost degree
lacking intelligence	(of especially persons) lacking sense or understanding or judgment
in a state of mental numbness especially as resulting from shock