'unacceptableness' originates from the prefix 'un-' meaning 'not' and the word 'acceptable', which comes from Latin 'acceptabilis', where 'acceptare' meant 'to take or receive'.

'unacceptableness' changed from the Middle English word 'unacceptable' and eventually became the modern English word 'unacceptableness'.

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