One derivation of this term comes from perception of society that people who devote large amounts of their time to lifting weights in the gym must not be very bright, due to the excessive nature of the weight-lifting activity. There is further perception that a bodybuilder or weightlifter has other serious shortcomings that result in him/her feeling the need to devote their lives to changing their body shape by rigorous exercise. Thus we call someone a meat head due to the assumption that their head is full of meat or muscle, and not brain tissue. Meathead is thus synonymous with stupid.
This occurs when your father, or another man, or woman with a dildo, or your mum with a dildo, entered your mothers vagina while she was pregnant with you which led to the impression of the penis/dildo being left in your soft skull. This then becomes a feature of your skull. It can be useful for storing food or as a flower vase.
'Look at that big whole in his head mummy'
'Don't stare, hes got cok in de head syndrome'
A person that drinks the alcoholic beverage "joose" way to often, as in a daily beverage and has to have 2 or more a day. Is often referred to as a Joose head. Or just has drank to much "joose".
"Damn man, I've had 3 Joose's already. I'm ready for another one!"
"Dude your a damn Joose head"