'unthinkable' originates from the prefix 'un-' meaning 'not' and the word 'thinkable', which comes from the Old English 'þencan', meaning 'to conceive or imagine'.

'þencan' transformed into the Middle English 'thinken', and eventually became the modern English word 'thinkable', with 'un-' added to form 'unthinkable'.

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