An erection on a male who has only one testicle. Typically displayed in online chats or ASCII form as a U plus a seemingly appropriate number of pipes (|) followed by a D to indicate the head. The head may be preceded by two or more slashes (\) if a pulled back foreskin is present.
Man, I could totally see Lance Armstrong's One Ball Boner in that Tour de France spandex.
by stbn January 03, 2010