Little_sister : Hi! (bumps shoulder and hugs) OMG! You are not going to believe what happened today, OMG! OMG!, OMG!
Older_brother : meh...
Little_sister: Remember that guy that ... (5 minutes later) ... can you believe it, OMG!
Older_brother: AAAARg (just killed by head shot) <stares at sister> GOOMF!
Using this expression seriously will only earn humility; it sounds too goofy.
Another way of saying, "Get out my grill."
-Normal person, "Goomf, you asshole."