Adjective: A nicer way of calling someone stupid. Can be used when talking about someone that's right next to you, or telling it straight to their face.
Jill: Nah, he's just UD.
Pam: Isn't Ray William Johnson a senator or something?
Mike: ....you're so UD.
Truant: "Shut yer hole or I'll UD yer ass until you cry."