Wiktionary
en femme
adv. (context LGBT of a trans or genderqueer person English) Dressed in feminine clothing.
Wikipedia
En femme
The term en femme is a lexical borrowing of a French phrase. It is used in the transgender and crossdressing community to describe the act of wearing feminine clothing or expressing a stereotypically feminine personality. The term is borrowed from the modern French phrase en femme meaning "as a woman." Most crossdressers also use a female name whilst en femme; that is his "femme name". In the cross-dressing community the persona a man adopts when he dresses as a woman is known as his " femme self".