Origin and history of nitwit

as an answer, "nothing, none," 1789, from German nix, dialectal variant of nichts "nothing," from Middle High German nihtes, from genitive of niht, nit "nothing," from Old High German niwiht, from ni, ne "no" (from PIE root *ne- "not") + wiht "thing, creature" (compare naught). By extension, as an adverb, "no."

"mental capacity, the mind as the seat of thinking and reasoning," Old English wit, witt, more commonly gewit "understanding, intellect, sense; knowledge, consciousness, conscience," from Proto-Germanic *wit-, which is reconstructed to be from PIE root *weid- "to see," metaphorically "to know" (also compare wit (v.) and wise (adj.)).

The meaning "ability to connect ideas and express them in an amusing way" is recorded by 1540s, hence "quickness of intellect in speech or writing" (for nuances of usage, see humor (n.)). The sense of "person of wit or learning" is attested from late 15c.

To be at one's wit's end "perplexed, at a loss" is from late 14c. Witjar was old slang (18c.) for "head, skull." Witling (1690s) was "a pretender to wit." Witword was "testament." Also in Middle English of the sensitive faculty generally, and sensory impressions, as wittes five for the five bodily senses (c. 1200).

Germanic cognates include Old Saxon wit, Old Norse vit, Danish vid, Swedish vett, Old Frisian wit, Old High German wizzi "knowledge, understanding, intelligence, mind," German Witz "wit, witticism, joke," Gothic unwiti "ignorance."

A witty saying proves nothing. [Voltaire, Diner du Comte de Boulainvilliers]

Wit ought to be five or six degrees above the ideas that form the intelligence of an audience. [Stendhal, "Life of Henry Brulard"]

"foolish, stupid and ineffectual person," by 1934 in British slang, popular 1950s-60s, it crossed to U.S. with BBC comedy programs. It probably developed from twit (v.) in the sense of "reproach," but it may be influenced by nitwit.