'forties' originates from Modern English, specifically the word 'forty' with the plural suffix '-s'.

'forties' derives from 'forty' (Middle English 'forti' or 'forty'), which in turn comes from Old English 'feowertig' (from Proto-Germanic *feowwōrti).

Originally formed simply as the plural of 'forty' to denote the number; over time it has also come to be used idiomatically to refer to the decade (the 1940s) or the age range 40–49.