a girl who's pussy has continuously been fucked the living daylights out off. The term "bus" is used to represent old school buses that have had stains, burns, tape, and beat up insides. We've all been in school buses and you know how used they've been.
"Yo wassup. so i heard u got on "the bus" the other day how was it?"
"Man that chick is a BUS, everyones been there"