'unquestionable' originates from the prefix 'un-' meaning 'not' and the word 'questionable', which comes from the Latin 'quaestionabilis', meaning 'able to be asked'.

'quaestionabilis' transformed into the Old French word 'questionable', and eventually became the modern English word 'questionable', with 'un-' added to form 'unquestionable'.

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