'sex' originates from Latin, specifically the word 'sexus', where 'sexus' meant 'division, gender'.

'sex' changed from Old French 'sexe' (from Latin 'sexus') and eventually became the modern English word 'sex'. The plural 'sexes' is the regular English plural formed from 'sex'.

Initially, it meant 'biological division into male and female'; over time the word also acquired senses related to sexual activity and social/identity senses, while retaining the biological meaning.