Green’s Dictionary of Slang

filly n.1

[SE filly, a young woman]
(US gay)

1. an effeminate male homosexual.

[US]B. Rodgers Queens’ Vernacular 73: stereotype effeminate homosexual [...] filly (because a filly can be mounted).

2. a passive lesbian.

[US]B. Rodgers Queens’ Vernacular 137: the passive lesbian [...] filly.