A person with facial features that are abnormally close together and a misshaped head that is wider than it is high (resembling a hamburger). Due to the inadequate spacing between their eyes, nose and mouth, hamburger heads typically have huge "never-ending" foreheads and their eyes are actually located on the lower half of their head (below the equator, if you will -- where most peoples' noses are).
Whereas the average person's face takes up approximately 70% of the front of their head, a hamburger head's face takes up far less real estate, averaging approximately 30% of the front of their head.
Steve: Dude, look at that ugly guy! His head is so flat and wide and his face is all scrunched together. I think I'm gonna puke!
Billy: Oh, that's Frank the hamburger head.