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" /ɑː
s/ are now very different.
-"Joe, how do you say donkey in Spanish?"
-"Burro."
-"How would you spell it?"
-"B-U-
R-
R-O-W."
-"... it seems that you
don't know your ass from a hole in the ground."