When someone has had anal sex and that persons partner ejaculated inside of her (or him in the case that is was gay sex.) The person is left with jizz dripping from her butt the next day.
Bro: AWW I gave Beth the WORST drippy butt last night!
Bro 2: Really??
Bro: Yeah bro, look, her jeans are all wet.
When a male busts a large load of seamen into a femalesanal cavity, seamen then seeps out of her ass, thus impregnating her, and there for, her offspring will be forever known as a buttdrip.