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.
When one's ass, usually larger female's, stick so far out horizontally that it may be used as a shelf to place items on.
"Hey dude, have you seen my drink?"
"Oh broski, I left it on that bitch's shelf ass ahead of us in the line in Wal-Mart."
"What the fux, bro."
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!
When someone is really really fat and their ass sticks out and it looks like a shelf. You can put pictures, lamps, or even write a paper on it.
Fat big assed people, usually woman