Green’s Dictionary of Slang

bumper n.3

[ext. of bum n.1 (1) + echoic of the buttocks hitting a hard surface]

1. [late 19C] (US) a trunk.

2. [20C+] (US/W.I.) the buttocks [? play on keister n. (1)/keister n. (5)].

3. [1940s+] usu. in pl., the female breasts; occas. buttocks.

4. [1970s] (US) the vagina.

In compounds

bumper kit (n.)

[1990s+] (US black) a woman’s posterior or buttocks.

In phrases

bumper to bumper (adv.)

[1970s+] (US lesbian) of lesbians, vagina to vagina, said of dancing or sexual intercourse.