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.