1. The wife of one's grandparent's brother.
2. The sister of one's grandparent-in-law.
My great-aunt-in-law is a good person.
by Elivaldeq November 24, 2019
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fanny is a euphemism for the buttocks. So the exclamation "My Aunt Fanny!" or "My great Aunt Fanny!" is to say
"My ass!" as something of a perturbed rebuttal or reply
"I think YOU dented my car."

(surprise and denial) "My great Aunt Fanny! I did not!"
by theguywhobrokethebank November 2, 2013
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