meaning that it is 'of the
story' or
something that could/did ACTUALLY happen in a
story, used by fans and fanfictioners
Izzi: Gosh, Marrie, you really like the Harry and Ginny pairing for Harry Potter...
Marrie: Yeah, but it's totally cannon.
Izzi: I know, I read the seventh book
Jen: OMG!
McGee and Abby are such a cute couple.
Dan: And it's cannon, have you
seen the latest NCIS episode?
Jen: I know, Abby totally kissed
McGee's head!!