1Used in various phrases expressing dismissive or disbelieving contradiction of what has just been said. Especially in "my Aunt Fanny!" and "tell that to my Aunt Fanny" and variants.
2In similative expressions as the type of someone ignorant or incompetent, or (more generally) someone or something unknown, irrelevant, etc.
1920s; earliest use found in Joseph Patrick McEvoy (1895–1958). From aunt + the female forename Fanny, perhaps as euphemistic alteration of my arse!.