Cockney rhyming slang. Note that this is really 'arse' and not 'ass' - reflecting the Cockney pronunciation. In London and the south east of England 'pass' is pronounced as 'parse'.
The term was used in the best of the Carry On series of films - Carry On Up The Khyber, 1986. Or, if not the best, certainly the one in which screenwriter Talbot Rothwell managed to squeeze in more bawdy puns than any other. It featured Sid James as Sir Sidney Ruff-Diamond and Kenneth Williams as Rhandi Lal, the Khasi of Kalabar.