'age-defier' is a modern English compound formed from 'age' + 'defier'. 'Age' originates from Old French 'aage' (later 'age'), ultimately from Latin 'aetas'/'aetatem' where 'aetas' meant 'period of life'. 'Defier' comes from the verb 'defy', via Middle English and Old French, where the verb carried the sense 'to challenge' or 'refuse to obey'.

The element 'age' comes from Latin 'aetas' → Old French 'aage' → Middle English 'age'. The verb 'defy' developed from Old French forms such as 'desfier' / 'defier' and entered Middle English; the agentive form 'defier' (one who defies) combined with 'age' in modern usage to create the compound 'age-defier', primarily as a coinage in 20th–21st century marketing and popular usage.

Originally, 'age' referred simply to a length or period of life and 'defy' meant 'to challenge or resist'. Over time the compound came to mean specifically 'one that resists the effects of aging' (often with promotional or figurative nuance).