Green’s Dictionary of Slang

absolutely nothing at all.

1942		G. Kersh Nine Lives Bill Nelson 63: You get away with S.F.A. [...] So mind your eye.
1947		P. Larkin letter 26 Feb. in Thwaite Sel. Letters (1992) 136: It seems to me I am wasting a lot of energy doing s.f.a. at present.
1983		J. Byrell (con. 1959) Up the Cross 35: ‘Cobber’s offered to train it for a whole year for essefay’.
1996		J. Byrell Lairs, Urgers & Coat-Tuggers 155: Caligula Perkins (who didn’t get called that for essefay) seems to light up.