the act of to women (lesbians or bi) having sex by bumping there vagina's against eachother.
dude, i caught my ex and my mom clam bumping last night
The illusory idea that homosexual women may have sex by bumping one female's vaginal labia against another female's vaginal labia. A slander. Other forms: clam bumper(s).
"Those dikey chicks are off to go clam bumping."