Yup, and: "---chiefly U.S.; from dial. variant pronunciation of arse (q.v.). The loss of -r- before -s- attested in several other words (e.g. burst/bust, curse/cuss, horse/hoss, barse/bass). Indirect evidence of the change from arse to ass can be traced to 1785 (in euphemistic avoidance of ass 'donkey' by polite speakers) and perhaps to Shakespeare, if Nick Bottom transformed into a donkey in 'A Midsummer Night's Dream. (1594) is the word-play some think it is."