Faf stands for "Funny As Fuck" or "a faf" when used to describe something funny.
Jeff: Wanna hear a joke?
Stan: sure :p
Jeff: Why do women make better soldiers?
Stan: why lol
Jeff: Cos they can bleed for a week and not die.
Stan: ur jokes r a faf xD
"I was trying to work things out with my best friend, but this bitch was being faf."
"I hate bitches who are faf"
(Fat ass fucking stroke) freaking out over nothing
I’m gonna have an fafs if I don’t have at soon