Sweet Fanny Adams is the original meaning -
'In 1869 new rations of tinned mutton were introduced for British seamen. They were unimpressed by it, and suggested it might be the butchered remains of Fanny Adams. "Fanny Adams" became slang for mutton or stew and then for anything worthless – from which comes the current use of "sweet Fanny Adams" (or just "sweet F.A.") to mean "nothing at all".'