From Odd Wisconsin:

Cheeseheads:

Because of Wisconsin's reputation for cheese, the term 'cheesehead' was coined to refer to Wisconsin residents, and it often was used in a derogatory manner. In 1987 Ralph Bruno of Milwaukee transformed the term into a symbol of pride when he crafted a wedge-shaped cheese hat from his mother's sofa cushion and wore it to a Brewers game at County Stadium. Fans flocked to Bruno inquiring about the strange hat and a business was born.

Manufactured cheese hats soon appeared on Wisconsinites' heads, especially at Green Bay Packers games. National television coverage of Packer fans helped the cheesehead hat become an instantly recognizable icon of the state.