1) When you be with yo girl, and you be working dat pussy so right that she be so wet, it shoots out like a jet. And you get to rocking that sweet punani so hard that her laffy taffy be slapping all on yo junk and flopping all over da bed.
2) When a man brings his female lover to such a state of arousal, that her vaginal canal produces a surplus of the lubrication required to facilitate intercourse. The male then proceeds to penetrate his partner so vigorously that her labial folds begin to "flop."
Note: Especially popular with women from the greater Pittsburgh area. Traditionally, the female would wear a small pair of furry booties on her feet while engaged in the aforementioned act.
Kent: Damnnn girl, you be so wet, I gots to clean up this mess.
Nicole: Boi, why you all about moppin', let's get to lippy flopping!