'unspent' originates from the English prefix 'un-' (Old English 'un-') meaning 'not' combined with 'spent', the past participle of 'spend' (from Old English 'spendan' meaning 'to pay out, to use').

'un-' + past participle 'spent' was formed in Middle English by combining the negative prefix with the past participle of 'spend' and developed into the modern adjective 'unspent'.

Initially, the formation signified 'not paid out or used' (especially money); over time the sense broadened to cover time, resources, and other things that remain unused, but the core meaning remains similar.