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The extra flap of skin that resides on the taint side of the butthole. It aids in stretch, so that the butthole can pass stool and accept foreign objects. Sometimes this is referred to as a butt clit.
I licked Johnnie's butthole yesterday and got his filbin stuck between my front teeth.

Dang, Kendra's filbin is bigger than my dick!
by nff123 August 01, 2011
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