This is a term which describes a broad high school stereotype. It can be used in a derrogatory manner (although not always) it's the sort of thing you might catch the so-called 'geeks' calling you as you drive way too fast out of the parking lot in order to get home in time for the 'OC'.
"Those idiots may have switched to wearing trucker hats but they'll always be bowheads to me."
"The only thing worse than watching George Bush speak is seeing a bowhead Republican flaunt daddy's bank account."
Professor Ashman: "Fifty bowheads and the football team. Again."