May also be defined as "Zombie, Oh My Fucking God!". Used as an expression of astonishment and fear when you spot a zombie and get terrified by what you see (the undead walking in the streets is not a pleasant discovery). May also be used as an expression of a feeling similar to the one you would get by the awesomeness, astonishment and fear you would get if you saw a zombie (more usual on a daily basis).
Ex. 1: Theoretically correct use:
Fred: *spotting a zombie* "ZOMFG!"
Ted: "Where is my freaking shotgun when I need it?!"
Jade: "Run for your fucking lives!" *screaming and running*
Ex. 2: Used as a expression:
Fred: "Zomfg! Check out that monster truck!"
Ted: "Wow, that's cool!"
Jade: "I think it's kinda scary ..."