'unimaginable' originates from the prefix 'un-' meaning 'not' and the word 'imaginable', which comes from the Latin 'imaginabilis', meaning 'able to be imagined'.

'imaginabilis' transformed into the Old French word 'imaginable', and eventually became the modern English word 'imaginable', with the prefix 'un-' added to form 'unimaginable'.

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