The origin of flappins began in Rugby 2007 when sarah flapped out on the sofa after a smoke. Since then, Flappins now flaps out anywhere and everywhere, whether on a train, up a tree, in the fridge,etc.
The act of a female with obvious labia wearing short shorts, underwear, swimsuit, etc., and in a position to where said labia is exposed in some way. Mostly frowned upon by the male gender, although when tolerated, strong sexual arousal is common.
Guy 1: Check it out dude! That chick with the booty shorts is totally Flappin'!