The degree of which a chick's ass sticks out in relation to the angle of her back.
For example, if a chick has a 90 degree slope angle, she has a huge ass. If she has a 135 degree slope angle, she has an athletic ass. If it's 180 degrees, she has no ass!
Guy1 "Look at that bitch's ass!!!"
Guy2 "Wow!! Nice slope angle!!"
Guy1 "Look at that bitch's ass!!"
Guy2 "Yeah!! Her I could tuck my beer in her slope angle! What a fatass!