PFK stands for Poor Football Knowledge and refers to someone who has a lack of knowledge about something that is pretty obvious in the world of football. It also refers to someone who is told a lie about something football related but believes it is true, otherwise known as a "Garner"
Person 1 - Ronaldinho is bangin in the goals recently isnt he
James - Yeahh, hasnt he scored quite a few for Argentina aswell?
Person 1 - PFK!! He plays for Brazil u arsehole!
Person 1 - Did you hear Marcus Vivian Foe has signed for Chelsea for 10.5 million?
James - yeahh, he scored loads last season didnt he?