'unregister' originates from the prefix 'un-' meaning 'not' or 'reverse' and the word 'register' from Latin 'registrare', meaning 'to record'.

'unregister' evolved from the combination of 'un-' and 'register', which was used in Middle English as 'registren'.

Initially, it meant 'to remove from a list', and this meaning has largely remained the same in modern usage.