When two, or more (if there's a lot of chemistry) people's ideas combine to make another idea.
My friend and I were having some difficulty figuring out how to stop skipping class in the morning, so we decided to have some idea sex and out came a flawless idea. Just drop the class in the first place.
When someone sends you a text pointing out something that's wrong or doesn't quite fit in the scenario, but at the same time you act as if you never got a text because it's rather personal.
Yo we were on the bus with this guy we just met and he was a little too social, so my friend Novian sent me a sneak-attext saying not to mention where we were headed off to when we got off. If he were to follow us, things would've gotten real awkward.