Green’s Dictionary of Slang

a joke corruption of SE colloq. phr. fair enough.

1981		P. Wright Cockney Dialect and Sl. 99: Fairy snuff ‘fair enough’ is certainly a true rhyme.
2001		‘Dr John Dee’ ‘Black Eye’ 25 Aug. on BlackArab.org 🌐 Dave Harper then made the decision that we would not be allowed in. Fairy snuff, it’s his decision to make, but the one thing that irked me was his decision to talk to us like we were stupid.