Earlier quoted context omitted.
but, in certain quarters in Great Britain, arse is pronounced as if it had no R
An inherited Americanism in itself.
> By the 1770s, postvocalic /r/-less pronunciation was becoming common around London even in formal educated speech. The English actor and linguist John Walker used the spelling ar to indicate the long vowel of aunt in his 1775 rhyming dictionary.[4] In his influential Critical Pronouncing Dictionary and Expositor of the English Language (1791), Walker reported, with a strong tone of disapproval, that "the r in lard, bard,... is pronounced so much in the throat as to be little more than the middle or Italian a, lengthened into baa, baad...."[8] Americans returning to England after the American Revolutionary War, which lasted from 1775 to 1783, reported surprise at the significant changes in the fashionable pronunciation that had taken place.[16]
...
> The loss of postvocalic /r/ in the British prestige standard in the late 18th and the early 19th centuries influenced the American port cities with close connections to Britain, which caused upper-class pronunciation to become non-rhotic in many Eastern and Southern port cities such as New York City, Boston, Alexandria, Charleston, and Savannah.[9] Like regional dialects in England, however, the accents of other areas in the United States remained rhotic in a display of linguistic "lag", which preserved the original pronunciation of /r/.[9]