Slang term for Masturbating. Made popular by Martin Lawrence in his 1994 stand up comedy special, "You so Crazy."
My homeboy's mom walked in on him chillin like a motherfucker while watching Debbie Does Dallas. She screamed just as he busted a nut.
The next level of chilling, or the intense version of chilling. Chilling so hard you might as well be on the bottom of the ocean. Not just sitting in a chair conversing with friends. Slouch down, shut your mind off, and sit there like a fucking rock. Best done in a big ass armchair. Eventually, someone will ask if you're spacing out or some shit, and you know what to tell them. "I'm chillin' like a motherfucker."
Katie: Ed, what are you doing?
Ed: *comes out of stupor* What?
Katie: Are you asleep?
Ed: Hell no. I'm chillin' like a motherfucker.