Green’s Dictionary of Slang

a masculine lesbian.

1941		G. Legman ‘Lang. of Homosexuality’ Appendix VII in Henry Sex Variants.
1950		Goldin et al. DAUL 36/2: Bumper (or Pussy-bumper). A moral eccentric in sex relations with women; a male oral copulator whose degeneracy extends only to women.
1965		Guild Dict. Homosexual Terms 6: bumper (n., obs.): A lesbian; used by heterosexuals of one who is thought to be active.
1982		Maledicta VI:1+2 (Summer/Winter) 132: Bumper (active tribade).