Ones (usually always girls) who may, or may have sexual encounters in a public - bathroom stall or public bathroom, in a public place. They do this because: they probably don't have any where else to go; it could be the quickest place to go at that time. Or their just that much of a dirty slut (smut), that they just have to "get it in", & it doesn't obviously matter where they have sex; as long as they get it over with.
Girl: "Whatever, they're Stall Sisters anyway. (;"
Girl's Bestfriend: "Oh, yeah ? Haha, whats that ?"
Girl: "Loretta fucked in a b-stall. Elaina looks like she was born in one. & Lacey smells like one. Therefore, they're Stall Sisters !"
Girl's Bestfriend: "Lmfaaaao !!!"