When you screw yourself by saying something intended for amusement, and then realized you just completely made a fool of yourself or said something that would get yourself in trouble. This is also know as "shooting yourself in the foot" or just a "self shoot"
friend: "hey parker, my mom is in the hospital with a severe case of meningitis but i still think she will be okay."
parker: "Let me know when she dies *laugh*"
Student: "....ah....i'm not kidding"
parker: oh. (wide eyes. nothing to say. realizing he just did a self shoot)