FC short for F-B-o-C means Fucking Cocks, or Fucking Bouncing on Cocks, commonly used by individuals adept in H-M, or the Hand Method. It is not uncommon for those who use FC to currently be engaging/planning to soon engage in Fucking Cocks, or Fucking Bouncing on Cocks. Grammatically, FC can be used to express how impressive/amicable a situation/thing is by heterosexually challenged individuals.
"TRUE! THAT'S MAKING ME FC"
"Damn bruh, hes FC"
"wHaa? Sorry guys that was not FC."
"the... TUNNEL, is not FC at all."