ass lap

An ass lap is when a girl has an ass so big that it creates a make-shift lap in the back which can be used as a shelf, or a inconvenient place for a cat to take a nap.

Hey, I set my beer down for a second and now it's gone. Did you see where it went?" "Yeah man, it's over there, you set it on Amanda's ass lap and she walked away." "Oh shit man, my bad.