'forty' originates from Old English, specifically the word 'feowertig', where 'feower' meant 'four' and '-tig' (later '-ty') meant 'ten'.

'forty' changed from the Old English word 'feowertig' into Middle English forms such as 'forti'/'forty', and eventually became the modern English word 'forty' (from which the plural 'forties' and the numeric shorthand '40s' derive).

Initially it meant 'four tens' (a group of ten counted from four); over time it retained the numeric meaning '40' and extended to denote the decade or the age range from 40 to 49.