J.F.C. is the abbreviation of the popular phrase "Jesus F*cking Christ". Its most obvious definition is to be surprised by a recent event that was just way too f*cking unbelievable. Sometimes in events of even more epic lulz, it could be used in a way that hints that it could have possibly been foreseen, which it is thencompared to the phrase "that was ironic as f*ck".
Cranky Negra: Yo Jabron I been hearin' 'round here that you been messing wit mah gurlz.
Jabron: Yeah? SO WHAT? I do whateva I wantbecause I'm big and I pack dis gat right hurr.
Cranky Negra: You lie. You punk ass mutha f*cka.
Jabron whips out his gat and pops his ass!
You: Oh J.F.C.
Greg is such a John Mayer fan, he is going to go to the music store and mark out on the John Mayer signature stratocaster.
Greg worships Tom Brady so much he went to the barber to mark out with the Tom Brady hairstyle.