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

'intentional' evolved from the Latin word 'intentionalis', which was used in Medieval Latin to describe something done with intention.

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