'unreserved' originates from the prefix 'un-' meaning 'not' and the word 'reserved', which comes from Latin 'reservare', meaning 'to keep back'.

'reservare' transformed into the Old French word 'reserver', and eventually became the modern English word 'reserve', with 'un-' added to form 'unreserved'.

Initially, 'reserved' meant 'kept back', and 'unreserved' evolved to mean 'not kept back or withheld'.