Inside Joke and you're on the Outside. Used on someone looking for an explanation of an inside joke. Usually leads to further confusion, as it is often use as an inside joke of its own.
Guy: Jane sure is bootylicious.
Guy 2: That thing is like a watermelon!
Girl: What are you guys talking about?
Guy: I J and you're on the O.
A sexual act that takes place, where a male with a black glove fingers an underaged girl during her period. When the bloody glove is used as evidence in court, the defendant pleads innocent and pretends the glove doesn't fit him.
Aw shit, so that girl WASN'T 18? I might have to pull the OJ Simpson on my court date.