A girl that never has any idea of what is going on, relative to her surroundings, coursework, pop culture, politics, social problems, or as how to juggle.
"Oh snap, that chedderhead has a big wart on her forehead"
"See that chedderhead over there, she don't know shit!"
A mature cheddar head is someone who is very mature but acts really stubborn and stuck-up with it, quite snobby. They are mature in a really boring way with no sense of humour and take everything seriously.