The female equivalent of a tea bag. When a girl lowers her genitals onto the forehead, or top of head, of an unsuspecting victim.
Dude #1 - "Lauren passes out by 10 o'clock at EVERY party."
Dude #2 - "It's all good, Melissa just gave her a fish hat. When she finds a diamond shaped hickey on her forehead tomorrow morning, she'll learn her lesson."
When the pussy is so loose that you can fit your head inside, in order to accomplish this tast it is best to have a bald head that has been lubed up with hot butter.
I wore your grandmas fish hat yeasterday, it was so loose enough to fit two watermellons inside.