'unintentionally' originates from the word 'unintentional', which is derived from the prefix 'un-' meaning 'not', and 'intentional', which comes from Latin 'intentio', meaning 'a stretching out'.

'unintentional' evolved from the Latin word 'intentio', which transformed into the Old French 'intention', and eventually became the modern English word 'intentional'.

Initially, it meant 'not done on purpose', and this meaning has largely remained the same in modern usage.