When a girl (maybe guy) has no ass or booty
whatsoever. Drywall reference is used to describe a flat surface from the girls back straight down to her hamstrings (back of the legs). Does not look good in a bikini. More prevalent in people of Asian or Caucasian ancestry, not so much in those of African, South American, or Polynesian ancestry.
Guy 1: "Check out that chick, she's so hot! Cute face, nice stomach, big boobs, and toned legs."
Guy 2: "For real!... Oh, what! Gnarly! Check it out, drywall ass!"
Guy 1: "Snap, never mind..."