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2.
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 18, 2013
 
1.
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.
Pull over I need to wipe off this flap juice from that fart I just let out.
by NationW January 23, 2014