Green’s Dictionary of Slang

femme n.

also fem
[Fr. femme, a woman; note West Point jargon femme, a young woman]
(orig. US)

1. [late 19C+] a young woman [note synon. use in Kendall, Flowers of Epigrammes (1577): ‘Which are three ills that mischief men [...] The fem, the flud, the fire’].

2. [1930s+] (also femmie) an effeminate homosexual man.

3. [1940s+] a feminine lesbian.

4. [1960s+] the ‘female’ partner of a couple whether hetero- or homosexual.

In compounds

fem sem (n.)

[1900s] (US campus) a female-only college.