'unsuccessful' originates from the prefix 'un-' meaning 'not' and the word 'successful', which comes from the Latin 'successus', meaning 'an outcome'.

'unsuccessful' evolved from the Middle English word 'unsuccesful', which was derived from the Old French 'success', eventually becoming the modern English word 'unsuccessful'.

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