Green’s Dictionary of Slang

bumper n.7

[bump pussies under pussy n. + ext. of SE use; i.e. her aggressiveness]

a masculine lesbian.

[US]G. Legman ‘Lang. of Homosexuality’ Appendix VII in Henry Sex Variants.
[US]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.
[US]Guild Dict. Homosexual Terms 6: bumper (n., obs.): A lesbian; used by heterosexuals of one who is thought to be active.
[US]Maledicta VI:1+2 (Summer/Winter) 132: Bumper (active tribade).