'unforgivable' originates from the prefix 'un-' meaning 'not' and the word 'forgive', which comes from Old English 'forgiefan', where 'for-' meant 'completely' and 'giefan' meant 'to give'.

'forgiefan' transformed into the Middle English word 'forgiven', and eventually became the modern English word 'forgive'. The prefix 'un-' was added to form 'unforgivable'.

Initially, 'forgive' meant 'to give completely', but over time it evolved into its current meaning of 'to pardon or excuse'. 'Unforgivable' thus means 'not able to be pardoned'.