'unbelievably' originates from the word 'unbelievable', which is derived from the prefix 'un-' meaning 'not', and 'believable', which comes from 'believe', originating from Old English 'belȳfan'.

'unbelievable' evolved from the Old English word 'belȳfan', which transformed into 'believe' in Middle English, and eventually became 'unbelievable' in modern English.

Initially, 'believe' meant 'to have faith or confidence in', and 'unbelievable' evolved to mean 'not able to be believed'.