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A term used as a taunt for or indicator of people (usually males) who partake in the practise of Sharking (womanizing noticeable within a clique). When a fin is supposedly up, the person doing the mocking and/or sharking will raise a hand, rotate it vertically and place it above their head to symbolize the fin of a shark.

Typical attire is basically a flat cap, sometimes sporting a shark nickname ie The Dread.
Hey Josh, I saw what you were doing, fin's up mate

That dude is blatently sharking that girl on facebook, I'm gonna type fin's up and get at least five likes
by TPH Warpath November 20, 2011
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