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“Until the Victorian period, “ass” had no profane meaning and simply referred to what we now call the donkey. Because of the increasingly non-rhotic nature of standard British English, “arse” was often rendered “ass”. This usage was also adopted in America, which is why the word “arse” is not usually used in the United States. The age of Victorian propriety resulted in the rechristening of the horse-like animal, changing the name to “donkey” to avoid any improper inferences. Some people in Britain have adopted the American version in writing. Although before World War I they were similar, the British pronunciations of “ass” /æs/ and “arse” /??s/ are now quite different. While arse is commonly used in Atlantic Canada, west of the Ottawa river, ass is more idiomatic.” –http://en.wikipedia.org/wiki/Arse

