A foolish or stupid person; someone who is not very intelligent.
From 'nit' (a louse egg or something worthless) combined with 'wit' (intelligence or mental sharpness). The compound emerged in the 1930s as slang for someone with the intelligence of a nit.
A 'nitwit' contains a pun—it literally means someone with the smartness of a louse egg, but in the 1930s it was just meant as silly slang. It's one of those insults that's funny enough that people don't take it too seriously.
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