The noun meaning "foolish, stupid and ineffectual person" is first attested 1934 in British slang, popular 1950s-60s, crossed over to U.S. with British sitcoms. It probably developed from the verb sense of "reproach" but may be influenced by "nitwit".

The bolded part is probably what people are thinking about. In any case certain activisty folk aren't too fond of certain modern conceptions of intelligence (i.e., intelligence as universally measurable/comparable, lack of culturally conceived intelligence as subhumanizing, etc.), so maybe that's also playing a part?

idk my bff etymology