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!!