A person who is stupid or not very intelligent; someone who acts foolishly or has difficulty understanding things.
From 'dim' (not bright) plus 'wit' (intelligence, from Old English 'wit' meaning mind). This compound became common in the early 1900s as an informal insult for someone lacking mental sharpness.
The word 'wit' originally meant intelligence itself, and old phrases like 'to keep one's wits about one' mean to stay mentally sharp—so 'dimwit' is literally calling someone's brain 'not bright,' a creative insult!
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