Jacob B thanks for that. Thats about all I could find from the Forum search, but the humour relies on the American pronunciation of "ass", whereas Steve Roud's query referred to the British verse of "arse up high".

It appears that the query is on behalf of Terry Pratchett who wants to quote the lines in one of his forthcoming Discworld novels, but wants to find out if there is a copyright on the words.
It would be nice if somebody here at Mudcat could source the words and get an acknowlegment for Mudcat in the next Discworld novel!