'single-sex' originates from English, specifically the words 'single' and 'sex', where 'single' meant 'one, not double' and 'sex' meant 'gender'.

'single' developed via Middle English 'single' from Old French 'sengle' and ultimately from Latin 'singulus'; 'sex' comes from Latin 'sexus' (via Old French/Latin usage). The compound 'single-sex' is a modern English formation combining these two words.

Initially it meant 'consisting of one sex' and this core meaning has remained largely unchanged in modern usage.