A "flap-p-ie-cou-pap-p-y" is a very large vagina, in danger of the uterus falling out. It happens a lot in the old folks homes, but can be attributed to "whore", "slut", or "loose", woman...
"That flappycoupappy better have my money!"
"Tommy's Grandmother has a enormous flappycoupappy, and the the janitor isn't so happy..." :(
"I had a dream last night, about your flappycoupappy..."
"What'da ya say we go back to the loft and I'll sew up that flappycoupappy'uh yours..."
The action of slapping your junk/balls against your thighs to produce a sound.
Roommate 1: Dude, what the hell was that noise from your room last night!?
Roommate 2: Just me standing in front of the mirror slappin' out that flappywappins.
It is said of the situation where a person has the bad luck to make contact with his testicles against an undefined surface or object, intentioned or not.
Given the nature of the word, it is more appropriate to design cases where the interaction is made with a moving object, for example, a ball.
Although it is extremely painful for the victim, it tends to be considerably funny to people who witness it.
Today in the baseball game the pitcher took a nutshot; the baseball hit him in the nuts.
Man, I just watched the funniest nutshot video ever.