When ones penis tip protrudes through a hole in their underwear or clothing, either accidental or on purpose, resembling a sharkfin cresting the surface of the ocean.
Tim's rush to leave the bathroom left his penis tip dangling from his open fly, revealing his shark fin to the entire office.
Sneaking up behind someone and using four fingers jamming them into their clothed butt crack while screaming "Sharkfin" in order to disrupt theiron train of thought
I just totally gave that chick a Sharkfin when she was giving me a hard time
Inserting a hand (or hands) into a persons orifice. The hand must be oriented in a manner that the four fingers lay on top of each other for the "insertion". The thumb points upward to resemble a sharks fin.
Dick and Fanny were having some quiet time. Fanny grew excited, resulting in Dick delivering a "SharkFin".