Green’s Dictionary of Slang

a masculine lesbian.

1941		G. Legman ‘Lang. of Homosexuality’ Appendix VII in Henry Sex Variants.
1965		Guild Dict. Homosexual Terms 22: horsewoman (n.): The aggressive, masculine lesbian; used mostly by heterosexuals to describe the ‘horsey’ masculine clothing and gait of this type of lesbian.
1972		B. Rodgers Queens’ Vernacular.
1982		Maledicta VI:1+2 (Summer/Winter) 147: From them she might pick up and more to startle than identify with her sisters use words and expressions such as [...] horsewoman (he-she or butch bitch).