'unhealthy' originates from Old English, specifically the word 'hælþ', where 'un-' meant 'not' and 'hælþ' meant 'health'.

'hælþ' transformed into the Middle English word 'helthe', and eventually became the modern English word 'health', with 'unhealthy' forming by adding the prefix 'un-'.

Initially, it meant 'not in good health', and this meaning has largely remained the same in modern usage.