'unfasten' originates from Old English, specifically the word 'unfæstnian,' where 'un-' meant 'not' and 'fæstnian' meant 'to fasten.'

'unfæstnian' transformed into the Middle English word 'unfastnen,' and eventually became the modern English word 'unfasten.'

Initially, it meant 'to make loose or undo,' and this meaning has largely remained the same in modern usage.