'unjustifiable' originates from the prefix 'un-' meaning 'not' and the word 'justifiable', which comes from the Latin 'justificare', where 'justus' meant 'just' and 'facere' meant 'to make'.

'justificare' transformed into the Old French word 'justifier', and eventually became the modern English word 'justify', with 'unjustifiable' being formed by adding the prefix 'un-'.

Initially, it meant 'not able to be justified', and this meaning has largely remained the same in modern usage.