'unintelligent' originates from the prefix 'un-' meaning 'not' and the word 'intelligent', which comes from Latin 'intelligens', meaning 'understanding'.

'intelligens' transformed into the Old French word 'intelligent', and eventually became the modern English word 'intelligent'. The prefix 'un-' was added to form 'unintelligent'.

Initially, 'intelligens' meant 'understanding', and 'unintelligent' has consistently meant 'lacking understanding or intelligence'.