Fuckin' off fetti (or fetty) is pretty much throwing away money. E40 has a verse in a Big Sean track where he says, 'You liable to find me up in the MGM casino in the D / Fuckin' off fetti I coulda put on property'. In this verse he equates gambling at the MGM in Detroit to wasting or throwing aweay money, or spending it as though he doesn't care what happens to it.
Fuckin' off fetti/y could draw its roots from the term 'fuck you money' which most commonly means the kind of money where you can tell everyone to fuck off and still be totally cool. In other words, you have so much cash you don't need nothin from no one.
I was out last night and some dudes at another table were fuckin' off fetti buying ace all night.