It's when a woman has no booty at all just pure flat cakes. Some women are tricky with their noassatall and can take pictures to try and make it look as if they have ass, when they thought.
Some other tricks include women wear leggings, super tight jeans, or booty shorts to try and make it look like they have big butts. This is called the leggings illusion, when it looks like she has an ass from afar, but you get close and notice that she suffers from noassatall.
Cures for noassatall include butt implants, but pads(Nicki Minaj'ing it), or eating until you ass gets fat and that's not a real booty, just nasty fat cottage cheese ass. If you don't have an ass, you just don't.