from this site:Naval Terminology, Jargon and Slang FAQ fanny - at least about the time canned meat was introduced, was canned meat.

and from this site:word and phrase origin of the day

"Fanny Adams was murdered and mutilated in 1812, her body cut into pieces and thrown into the River Wey at Alton in Hampshire. Her murderer, one Fred Baker, was publicly hanged in Winchester. Young Fanny Adams's name, give wide currency, was adopted by sailors to indicate a particularly distasteful meal, since Fanny Adams had been disposed of in a kettle. In fact, when kettles came into use in the British navy they were dubbed Fannys as were tins or cans of meat. There is no doubt that Fanny Adams is the basis for the military expression Sweet Fanny Adams, meaning something worthless or nothing at all. "