A butt that sticks out so much you could use it for a shelf.
She had such a shelf-ass she could carry the groceries in without using her hands.
Where the back and butt of a person seem to intersect at a 90 degree angle forming a shelf. For which one can rest an arm, cup, or book, like a bookshelf.
DAMN! Check out that shelf-ass on that ho, I could rest a cup on it!