my aunt Fanny

My aunt Fanny is a British expression of disbelief or dismissal, equivalent to "nonsense" or "I don't believe it": Genuine leather, my aunt Fanny—this is plastic.

The phrase uses the fictional aunt as a stand-in for something obviously false.

Fanny being British slang for the female genitals gives the expression a mildly vulgar edge, though many speakers don't register this connection.