Basically an emphasis of for real. It comes from "fer serious" which is how people with bad grammar would say it. (Also "fo' cereal" or "fo' serious")
by Chris Leyva October 20, 2004
Tiimmy: My mom got hit by a flying magic ice cream truck with 26 legs and a robo reptar cereal box arm!!
Jimbo: are you Fer-cereal?!?!?!?!!!?
Jimbo: are you Fer-cereal?!?!?!?!!!?
by Rawrpeople April 25, 2010