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.
Can you define these popular missing words?
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.