Nobody really knows, but most people use the term as a meme to describe when either a long series of events leads up to something, or when someone "gets owned". Again, I don't really know why are what is was when it first came out.
Okay. I don't see it here. But ~shrugs~ there is a lot I miss. Mostly because I don't really pay a lot of attention to social interactions (a lot of the behaviour is just beyond my comprehension).