GFaC means Go Fuck a Cow. It is widely considered a step above the common insult 'Mother-fucker', due to the fact GFaC involves fucking a cow.
Roger: You are so annoying Melissa, why don't you just fuck yousfelf?
Melissa: Why don't you just GFaC?
Roger: Shit, I've been owned.
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Often used by pilots to as a synonym for 'flight'.
1) Let's go aviate in my new Piper Cherokee!
2) Today's weather ain't that good for aviating.