The word "ass" (usually marked as "vulgar"; the one that means "buttocks," "butt," etc.) comes from Sanskrit, one would think, since the old Germanic version is not a stand-alone, but has its colleagues in Latin, Greek, and Armenian.
Anyway, the Germanic original is ars; the modern German arsch; the current British pronunciation (to the best of my knowledge) is "arse."
I would have continued to assume that "ass" is an Americanism had I not encountered the word "asshole" in one of Douglas Adams' installments of the "Hitchhiker Guide." Now Adams was, when it came to language, a British patriot. He pronounced schedule "shedyule," was steadfastly opposed to splitting infinitives, called trucks lorries (and, I suspect, avoided words such as "elevator" and "faucet" as hopelessly Gallic, favoring instead such absurdities as "lift" and "tap"), and so forth. (I happen to agree with him on the infinitives, but that's beside the point).
What gives? The vowel is altered; the consonant is gone. To what end? Why? How?