'dimwit' originates from English, specifically a compound of 'dim' and 'wit', where 'dim' meant 'not bright, dull' and 'wit' meant 'mental sharpness or intelligence'.

'dim' descends from Old English 'dimm' meaning 'dark' and 'wit' from Old English 'witan'/'wit' meaning 'to know' or 'intellect'. The compound 'dim-wit' arose as informal/slang in early 20th-century English and became the established word 'dimwit'.

Initially the components referred separately to low light ('dim') and knowledge ('wit'); combined in slang they came to mean 'a person lacking intelligence' and that sense has persisted.