Originally, the other, not-usually-played side of a 78-rpm phonograph record. From way back in the day, whenever it was that two-sided phonograph records became popular.
As '50's slang: You turn my record over, baby. Like, on the *flip* side. (See "flip out" = "go crazyover, freak out over, be turned on by someone or something.")
The dark places where douchebags, posers, rejects, thieves, and other horrible people reside. It is known as a Flip-Side because it is the opposite of our reality of relativelydecent people.
"Hey, what's going on over there?"
"Oh, that's the Flip-Side, where the weird-o's go."
"Oh. Okay."
- An example of common education that should be shared among all people.