I have a work colleague who was stationed in the UK for a number of years, and got in inadvertent slang trouble with her then boyfriend's family. They were talking about bike riding, and she said it was great but her "fanny" really hurt. Dead silence, Here in north America it's slang for butt, but in the UK it's a quite vulgar term for part of the female sexual anatomy. That's why fanny packs in the US are called "Bum Bags" in the UK.