Ferserious is a two word add-on. The Fer is added to mean the followingstatement "you are more then fucking serious" so therefore would be FURSERIOUS, used instead of saying I'm Fucking Serious
Guy says, "Dude, I gotta confess, I slept with your wife on your honey moon." other guy says, "Are you furserious?!? I fucked your girlfriend Emily when you were in the Air Force."