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")
-Dude, I got free porn now.
-Fer cereal?! Awesome.
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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?!?!?!?!!!?