Never understood the word "arse". Just far inferior to the alternative "ass".
Zee English can't really say ass without sounding like a twat though, unfortunately.
Actually, I lie. It works in sexual situations (linguistically speaking, of course).
'can I stick it in your ass?'
In that context ass is a very plausible replacement for arse. Even the most nothernly laydee won't roll over on her belly after an ear full of 'wot iv I pt it n ur arse'.