Can sometimes be spelled fc, or F.C., but most commonly spelled as FC. FC is a group of hoodlums that wreaks havoc on innocent bystanders. If you see FC written somewhere they have probably struck there. They like to break into peoples houses and they doesnt afraid of anything. Thats why I bought a dog.
It means: if you need to check some place that you don't have vision at and you dont have any wards you have to just go there and look there by your own body. It may be very dangerous situation because enemies can be waiting there for you to come.
f/c means face check. In League it is used when you need to get vision and you don't have wards. So you just come there and check it (by your body) if its clear.