Greasy or sweaty hair matted to ones head after wearing a hat for a prolonged period (generally a baseball cap).
Dude, put you hat back on, you've got a serious case of hat hair.
Inadvertantly getting one's pubes stuffed into one's condom prior to sex. After sex, the condom is removed to reveal a sweaty mess of matted pubes also known as "hat hair."
I had to put the condom on so fast I didn't bother moving my pubes out of the way. After I blew it, I had a ratty nest of hat hair that was sore for three days.