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.