That has got me thinking, at what point did Americans begin to use 'ass' to mean rear-end? These changes between American English and real English are interesting wink An ass is a mammal of the equine variety; 'arse' is yer arse, innit.
The earliest citation in the OED of the use of 'ass' in that sense is dated 1860, but it's the kind of thing that would not have appeared often in print, so it probably dates back well before that. Ever since I figured out that the original word for posterior was 'arse' (it appears in Chaucer as 'erse,' pronounced the same way; the German cognate is 'Arsch') and that the use of 'ass' for that purpose is a confusion with the name of a kind of animal, I have regretted the conflation. But as an American, I am pretty much stuck with it. I use the word 'arse' when I can, but it just doesn't carry well on this side of the Atlantic.