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!
Noun: a butt characterised by the sharp right angle from one's back, to the top of their ass. The thighs blend straight into the booty, but somehow it is very distinct from the back. It looks like they sat down for so long that their ass flattened out. The sharp angle makes it look and act much like a shelf.