I had the fanny argument with a retarded girl from the UK, she was convinced it means pussy but I was brought up to believe a fanny was a butt.
That's probably because you weren't brought up somewhere that speaks English, you were brought up speaking American English?
Why did you draw the conclusion that the girl from the UK was retarded? Surely not because she uses the word "fanny" in the same way that everyone else in her country does?
I would suggest that anyone entering into an argument over something so trivial is retarded, especially as both positions are correct for their respective backgrounds.
Bottom line: If you're in the US it's a backside, if you're in the UK it's a cunt. Whichever country you're in has the correct usage for their own population, so arguing that one is wrong is only relevant if you're an English person in the US but still using fanny for a pussy, in that case you're using it incorrectly for & ignorantly of the common vernacular of the place that you're in. The same goes vice versa.
Wherever you are, their usage is correct for their country. The fact that it differs from one's own understanding drawn from their own country's vernacular does not make it incorrect, it makes your usage from your own country's vernacular incorrect when in their country, & theirs incorrect when in your own.
I'm English & I, from a pedantic point of view, prefer Mom as it is more closely related to the full word that it is shortened from: Mother.
Obviously I don't use Mom, I use Mum, but I would not be viewing an American as a retard for using Mom, nor would I if they referred to their arse, instead of their snatch, as a fanny.