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.