That itch you can't get out no matter what, no matter how hard you itch. Most people would do anything to get rid of the itch. See deadly itch.
Frank: What are you doing?
Joe: IT ITCHES!
Frank: What itches?
Joe, now dragging butt on ground: MY BUTT!! Make it stop!!!
Frank: Looks like you got a case of Butt Itch.