'female-only' is a compound of the adjective 'female' and the adjective/particle 'only' in Modern English; 'female' comes from Middle English (from Old French 'femele'), and 'only' comes from Middle English 'onli'.

'female' originates from Old French 'femele', ultimately from Latin 'femella' (diminutive of 'femina' meaning 'woman'); 'only' developed from Old English forms (such as 'anlic'/'ānlic') into Middle English 'onli', and the modern compound 'female-only' arose by straightforward combination in Modern English to describe restriction to females.

Individually, 'female' meant 'of or relating to women' and 'only' meant 'sole' or 'exclusively'; combined, they came to mean 'exclusively for females' with no major shift from the component meanings.