A person who usually shows some sign of intelligence (not completely necessary though), but has brief lapses where they act completely retarded and senseless.
Willie: My mother said it's the law that I need to have my permit for another year before I can get my license...
Dan: Willie, did you notice all of us only had our permits for six months? We're two years younger than you and have our licenses already.
Willie: Durrr, I didn't really think about it... *proceeds to stare off into the distance with a blank look while making indistinguishable noises*
dum-head to be dumb in the head. dumb+head = dumhead derrived from the term "dummyface"
When you feed a cat cheese and it pukes, you are a dumhead.
If your name is Tony Balez and between a preference of black and white, you choose "black with a little bit of white" you areindeed a dumhead.