The act of masturbating during a church service. Most likely, it's the man sitting in the back row all by himself. He's been caught chapelbating before, but they don't want to kick him out of the church because they're trying to be good people. So, no one sits around that one guy in the back.
Man #1: "I think I hear that guy in the back Chapelbating, bro."
Man #2: "Yeah, he's that guy that does that. No one sits around him because of it."