any surface at around ass level that serves as a make shift seat. Common types of ass shelves are window sills, arms of chairs, any sort of ledge, and if your creative your friends. As the word "shelf" suggests in the term, ass shelves serve as the perfect place to store your ass when a regular chair just won't do the trick.
Queefy: " Oh hey, who would have known that your arm would make such a great seat! I could sit here for days."
Hoeleen:"Don't look at me...I'm not ok with being your ass shelf!"
Queefy: "Lies...your loving every second of being my ass shelf ;) ! "
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.
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.