It is used to express a fair warning of potential involuntary violence towards it's recipient should their ignorant or out-of-line behavior not stop.
Also used when someone gets on your nerves and doesn't have the brain to understand.
It can be used in response to something that is unwelcomed, such as an invasion of personal space from strangers and coworkers.
"I go to work tramatized everyday... All these hoodrats need to GMFF, for real."
"That girl sneezed right behind me and was trying to lean all on me at work... and I was like, 'GMFF Miss Nasty.' "