'postexposure' is a compound formed from the Latin prefix 'post' (from Latin 'post', meaning 'after') combined with the English noun 'exposure', the latter tracing back via Old French and Late Latin to Latin 'exponere'/'expositio', where 'ex-' meant 'out' and 'ponere' meant 'to put/place'.

'postexposure' developed in Modern English as a compound (often written 'post-exposure') from the prefix 'post-' plus the existing English word 'exposure'. 'Exposure' itself evolved from Old French 'exposer'/'exposicion' and Late Latin 'expositio', derived from Latin 'exponere'.

The components originally meant 'after' (post-) and 'to put out/lay open' (exponere), but the compound has come to mean specifically 'occurring or relevant after contact with a hazardous agent' (e.g., medical or safety contexts).