'unbelievability' originates from the English word 'unbelievable,' which is derived from the prefix 'un-' meaning 'not,' and 'believable,' which comes from the verb 'believe' and the suffix '-able' meaning 'capable of.'

'unbelievable' changed from the Old English word 'belȳfan' meaning 'to believe,' and eventually became the modern English word 'unbelievable' with the addition of the prefix 'un-' and suffix '-able.'

Initially, it meant 'not capable of being believed,' and this meaning has largely remained the same in modern usage.