The original form was, of course, the American phrase, 'smart Alec'. The earliest recorded use of the cruder version - referring to the bottom rather than an actual name - was also the American word for that part of the anatomy rather than the British 'arse'. If anything, therefore, it is a "western saying" rather than a northern or southern one!
well arse is more northern thn the southern and american ass. So i guess smart arse if the r is pronounced is slighlty more northern.
Rocking it up liverpool stylee!
I'm not aware that southerners - though they do have some strange ways of pronouncing some words - say "ass" to denote the posterior. Of course, the saying could refer to an intelligent donkey.........
The word with an 'r' is - most directly - Teutonic in origin and common throughout Europe as a consequence...'arsch' in German, 'ars' in Swedish etc...though there is also a related word in ancient Greek. The matter of the north as opposed to the south of England is irrelevant.