The story
Coined in 1920 by the British physician and sexologist Havelock Ellis (1859–1939) after Chevalier d'Eon (1728–1810), a French diplomat, spy and soldier who presented as male for 49 years, then as a female for 33. Ellis had previously used the term sexo-aesthetic inversion.
Etymology from Wiktionary, CC BY-SA 4.0.