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.
A douche who only has one ball. This insult is usually shouted out in his general direction whenever he's around or whenever someone is talking about him behind his back.