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The crusty film that forms made of sweat that collects under a breast, usually a large breasted girl, or woman.. or anyone who has breasts large or saggy enough to cause and form pockets, if its large enough to hide items, its large enough to collect dirt sweat lint and small creatures. Liken it to swamp ass or belly button lint..
Dude, she flashed us and under flap fell out.

Much to his surprise and horror when he got done licking there, there was some under flap stuck in his teeth
by SLAA addict July 09, 2012

Words related to Underflap

dick genitals penis scrotum testicles
back side of the ball sack
"Bro, my underflap is so itchy!"
by koolaidjollyman September 28, 2009