Where two people remove their clothes and rub their bottoms together. Sometimes considered a sexual act, the art of bumming is also considered a ritual in some cultures. Commonly mistaken for anal sex, bumming is actually the craft of gentle contact between the arses.
Gary: "Wanna' have a bummin' sesh?"
Liz: "Yeah! Let's strip down and rub bums!"
When someone displays bum-like qualities.
Taylor: That guy just stole some free samples.
Bill: Ya, he's bummin'.