This came up in the Sherlock Games - alphabet thread today in a little discussion  between the Mollykins and moi. So as I am a saddo I am now posting some links so that people who want to find out more can do so. I am also going to keep my eyes peeled in the ACD stories to see if it makes an appearance. Back slang is much less well-known than Cockney rhyming slang but is interesting nevertheless. The word that started off this bit of research was 'yobbo' or 'yob'.

www.en.wikipedia.org/wiki/Back_slang

www.grammar.about.com/od/ab/g/backslangterm.htm