When your balls sag down low enough to cover your asshole like a flap and you let a wet fart leaving behind some juicy residue on your balls called flap juice.
by NationW January 23, 2014
The ooze that builds up on each corner of the mouth of that creepy neighbor that talks to much and prevents you from getting anywhere on time.
I ran into Mr Smith today and after an hour of him telling me about his cats urinary tract infection, I couldn't keep my eyes off his flap juice. spit gross
by Seadoochrissy March 19, 2013