Origin and history of dimwit

Old English dimm "dark, gloomy, obscure; not clearly seen, indistinct," from Proto-Germanic *dimbaz (source also of Old Norse dimmr, Old Frisian dim, Old High German timber "dark, black, somber"). Not known outside Germanic.

Of eyes, "not seeing clearly," early 13c. Of sound from early 14c.; of light, "not bright, faintly luminous," from early 14c. Modern slang sense of "dull of apprehension, stupid" is from 1892; the sense of "dull-witted" also was in Middle English (mid-13c.). Related: Dimly; dimness.

"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"]