I have come across this usage in the 18th episode of the 10th season of Friends. Here is the scene. It's at 2 minutes and 41 seconds.
Gate attendant #2: I'm sorry, you cannot go any further without a boarding pass.
Ross: No, no, no, but...
Phoebe: (screaming) RACHEL!!
(Rachel comes back to the gate.)
Rachel: Oh my God... What.. What are you guys doing here?
Phoebe: Okay, you're on.
No definition in this dictionary and others, it seems, doesn't fit.