'unfasten' originates from Middle English, specifically the prefix 'un-' meaning 'not' and the word 'fasten,' which means 'to make fast or secure.'

'unfasten' changed from the Middle English word 'unfastnen' and eventually became the modern English word 'unfasten'.

Initially, it meant 'to make something not fast or secure,' and this meaning has largely remained the same in modern usage.