'untighten' originates from the prefix 'un-' meaning 'reverse action' and the word 'tighten' from Old English 'tīhtan', meaning 'to make tight'.

'untighten' evolved from the combination of 'un-' and 'tighten', which itself came from the Old English 'tīhtan'.

Initially, it meant 'to make less tight', and this meaning has remained consistent in modern usage.