I love differences in word usage too although, as some examples above show, they can lead to some pretty embarrassing moments. Besides American/British differences though, a lot of it is just the change in meaning over time.
When I first heard about what 'fanny' meant here in the UK I was really surprised, since it was a very common name in the past (Jane Austen used it in her stories - although I suppose that is rather long ago
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). Unfortunately for me I use it quite often - in the American sense of course!
My driving instructor and I used to love talking about UK/US word differences and I'll never forget her telling me her sister in law used to live on Fanny Bush Lane.....
....That's here in Northeast England mind you.
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Evidently she refused to give her address out, even to the point of having deliveries made to a relative's house somewhere close by but on a less offensive sounding street. She's moved since then.... *snicker*