A term used in Eastern NC when one of your friends is nowhere to be found, because he/she is trying to avoid having to tell you they are doing something better than hanging out with you.
John: What's Dave doing?
Adam: Man, I tried calling him to; he's got to be shellin with his girl.
Mark: Man wtf
is up with Pat? I haven't heard from his ass in a week.
John: No clue dude he's a shell motherfucker.
Alice: What's Alyssa up to?
Mary: She didn't text me back either, I think she's shellin for some reason.