1.Intense, mostly pleasurable but possibly painful(for either party) anal sex. Mostly used as a humorous example rather than an enticing possibility.
2.A fierce display of buttsexery
3.also Roast Beef Sandwich(how Arbys got its name)
(word origins attributed to Zeb)
"You gotta have an ass of steel to handle my R.B.S"
"I thought R.B.S was cool til' the doctor had to sew up mysahara rip-dick"
Boy 2: "He has an awful case of R.B.S..."
Boy 1: "Oh... This is the 4th time this month!"
Boy 2: "...You've been counting?"
Boy 1: *walks away*