witless vs stupid

witless is not a noun while stupid is a noun.

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

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

witless (adjective)	stupid (adjective)
(used as complement) to the utmost degree	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