A person, usually a co-worker in your office, who consistently fouls up the bathroom to such a degree that he himself is embarrased to leave the stall until everyone has left in sheer terror. The only way you recognize the Phantom Crapper is by the smell of death emmitted from his stall, oh and by his shoes visible from under the stall door.
Ron: "Man, I made the mistake of taking a leak while the Phantom Crapper was in there. The stench was so overwhelming I had to cut it off midstream and evacuate."
Paul: "Any idea who it is? I will make sure not to use the restroom while he's in there."
Ron: "No idea, he never leaves the stall due to embarrassment. He's a Phantom Crapper."