A human being (or something like one) who is so retarded that they cannot speak, cannot walk by themselves, cannot eat by themselves, etc. They usually "communicate" through noises like "nnnnhh" or "eiirrr" or" "AOUEEEEOuuuu" and they shit their pants all the time so they usually fucking stink and they try to eat everything and don't listen to anyone. They usually have severe combinations of Downs Syndrome, Autism, Asperger's, and cerebral palsy.
Michael: "Hey nick, why is that kid moaning and rubbing shit on that trashcan?"
Nick: "Oh him? he's just one of those Ultra-Downy's."