Originally just the name for the donkey, this
word had no filthy connotations until about the late 1700s, early 1800s, when the pronunciation of true (English)
word for the backside, "arse", had changed enough to sound much like "ass", seeing as how the English often
don't pronounce the "r" when they speak.
To avoid any unwanted baggage associated with the
word, the name of the ass was changed to "donkey". As a result, the
word "arse" has disappeared from American vocabulary.
Although before World War I they were similar, the British pronunciations of "ass" /æs/ and "arse" /ɑːs/ are now very different.