"In British English, fanny is an offensive term for the vulva. In American English, it is an informal term for the buttocks. In British English, fanny is considered vulgar and could cause offense. In American English, fanny is arguably the most nearly polite of several euphemisms. In British English, the phrase fanny pack is bum bag in American English."
Its almost like we don't even speak the same language.....
It's not really considered rude, but it does mean vagina. And as with a lot of words like that, it's used as a generic insult; "You stupid fanny". You're not really calling them a woman's sexual organ, just making out that they've done or said something idiotic.