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tailfin can have two meanings: either the inner labia of a woman extending (too) far outwards or simply a gay man from Finland (tail Fin).
example: "man, she has such a sexy body but when she bent forward, I suddenly saw her huge tailfins hanging down like giant rhubarb leaves and I got frightened that these things might have teeth and eat up my dick"

another example:

Eric: Where´s this guy from? He´s talking like some kinda alien.

Zac: "He´s from Finland".
Eric: "He seems a little gay to me"
Zac: "Yeah man, he´s a real tail Fin (tailfin) if you know what I mean..."
by Pope on Dope April 11, 2015
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Protruding labia majora especially those that extend significantly below the rest of the vulva. Now considered unfashionable anatomy due to pubic shaving and sometimes corrected with labiaplasty.
She got her clothes off and her tailfins almost reached her knees.
by MBSea December 12, 2009
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