A fictitious condition that is used to insult someone who's backside is straight as a board. The person the joke is about usually is wearing pants that makes them look like they have no ass at all. The person may also end up with the nickname R.A.S. - Pronounced Rass.
Hey...Hey, Dude! Look at that chick!
Yeah, man. She looks like she's suffering from Receding Ass Syndrome.
Sister: What do you want?
Brother: I just wanted to see how bad your condition has gotten, R.A.S.