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 then compared 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 want because 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.
To emulate an idol or icon, through purchase of their tools or equipment, or through imitatation of their style or appearance.
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.