A person who is naturally dumb/stupid or clumsy can be said to have Danny Jones Syndrome. The kind who accidently walks into things or drops important objects. Sounds naturally dopey when they talk. Name taken from the guitarist of British pop band McFly, who could be said to have the original Danny Jones Syndrome.
My Dad's such a 'tard, he forgot my Birthday again! He must have a case of Danny Jones Syndrome!