What I meant was I never heard it in the context being discussed now, as a substitute for the posterior. However, I am sure it is the diminutive for Frances.
I am confused now. It is OK to call somebody Fanny if it is her name, but to call someone that if it is NOT their name is rude.I assume her name is Fanny? In which case, no - because it her name, and doesn't refer to a lady part.