'nitwit' originates from English, a humorous compound of 'nit' (meaning 'louse egg' or 'something very small') + 'wit' (meaning 'intelligence' or 'mind').

'nit' long meant a louse egg or something very small; 'wit' comes from Old English roots meaning 'knowledge/mind'. The coined compound 'nitwit' appeared in English in the early 20th century as a jocular term for someone with very little intelligence.

Initially it meant someone with the intelligence of a 'nit' (i.e., very little); over time the sense remained essentially the same, used informally to call someone foolish.