'unbelievable' originates from Old English, specifically the word 'belȳfan,' where 'un-' meant 'not' and 'belȳfan' meant 'to believe.'

'belȳfan' transformed into the Middle English word 'bileven,' and eventually became the modern English word 'believe,' with 'unbelievable' forming as a negation.

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