Half of the time, these people are not really unintelligent, but are just responding to a situation with lack of common sense.
People are usually referred to as dumbheads when the are drunk or do not agree with you.
Nicole: no, i cant. im going out with josh.
Jawhara: DUMBHEAD! now who am i gonna go with. your real dumb, and you smell oddly!
Nicole: dumbhead, dont call me that. i have a 4.0!