A clown, prankster, jokester. Term originates from the "Jokerman Font" typeface, found predominantly on Windows-based computers.
You're a jokerman. A complete jokerman.
A nickname for someone who has something unfortunate happen to them. The unfortunate incident is usually embarrassing, and always relatively trivial. Example incidents: losing your queen in a game of chess, getting taken advantage of in some way, saying something stupid in front of a large group etc.
Person1: "I locked myself out of the house last name"
Person2: "Hah! Gnome Chumpsky. How'd you get back in?"