When a girl (most likely an underaged teenage girl on a social networking site) takes a picture in the bathroom mirror, with her back to the mirror, and her butt seated on the sink, amplifying the rotundness of her butt. Often used to deceive potential men who would pursue sexual relations with her.
David: Broo! did you see that picture Kayla put up as her profile? I never noticed her ass looked that big before!
Brian: dude, it's not.. it's a classic case of sink booty.