'unprestressed' is formed in Modern English by the negative prefix 'un-' attached to 'prestressed'; 'un-' ultimately comes from Old English 'un-' meaning 'not', while 'prestressed' derives from the prefix 'pre-' from Latin 'prae' meaning 'before' combined with 'stress' (from Middle English 'stresse', via Old French, ultimately from Latin roots related to 'strictus').

'prestressed' developed from the compound 'pre-' + 'stress' in Modern English to denote something stressed in advance; the word 'unprestressed' arose by adding the productive Old English negative prefix 'un-' to create the negated modern term 'unprestressed'.

Initially the components conveyed 'before' (pre-) and 'pressure/tightening' (stress); the compound 'prestressed' specialized into a technical term for applying tension or compression in advance, and 'unprestressed' came to mean 'not subjected to that process' with little change in core meaning.