When you masturbate to a porno within a porno.
For example, when you are watching a porno and suddenly realize that the porno that the characters in the porno are watching is hotter than the porno that they themselves are doing.
Creeper 1: Dude, did you watch Ass Adventure IV yet?
Creeper 2: Yeah, but I don't remember anything from it. I ended up doing some metamasturbation when the girlfriend put on that video in the living room to get herself in the mood.