An expression of excitement, boredom, disgust, anger, or other's stupidity/incompetence.
Ken: Fo schnizz! I just got a high score on Alien vs. Predator 3!
Emily: Fo schnizz, dude.
Georgina: That alien fight scene was so fo schnizzin' disgusting!
Ham: Why the fo schnizz are you caring about Ken and not me?
Lara: Fo schnizz, Ken. Fo schnizz.