The act in which a girl gives you blue balls and you are so horny that you jerk off on the drive home.
Boy 1: The principals daughter gave me blue balls so bad I had to rub one off on the way home.
Boy 2: That’s called a shark fin!
When your boobs are the shape and depth of a sharks pectoral fins. Thin triangular slices of skin pointing to the floor. If one were to ejaculate on such deformities I guess you could call that shark fin soup
When she gets naked her boobs remind me of a sharks pectoral fins, but thiner, so shark fin slices