An acronym of "Fake as fuck", a term popularised by rapper Del Tha Funkee Homosapien in his 'Both sides of the brain' album (2000), denoting his view of two-faced people who say one thing to your face, and another behind your back.
"If you're brown nosin' in the industy (you fake as fuck)
Wanna pretend that you're a friend to me (you fake as fuck)
Funny style record companies (you fake as fuck)
Weak ass MC wannabe's (you fake as fuck)"
You know that dude Jackson? How he's always smiling and slapping your back, telling you how rad he thinks you are? Yeah, that's faf. He hates your ass.