"An [un]usual kind of slang, known as back slang, evolved in England. One of the places it flourished was in butcher shops, where it allowed the butcher to order his assistant to bring out the old piece of meat for this customer. A word was coded by writing it backwards and trying to make a sensible pronounciation, although certain sounds like "th" didn't actually get reversed, and extra vowels were inserted as necessary. In some cases, syllables were added or dropped, vowel sounds modified, or a single letter, such as "h", became pronounced."
I have heard that BackSlang was more strongly associated with the docklands and Rotherhithe, but I don't know whether that's really true. From the many references in Mayhew's works, it seems to have been largely a costermonger's slang.
Mayhew on Back Slang
"Again, we have the ‘Coster-slang’, or the language used by the costermongers, and which consists merely in pronouncing each word as if it were spelt backwards:—’I say, Curly, will you do a top of reeb (pot of beer)?’ one costermonger may say to the other. ‘It’s on doog, Whelkey, on doog (no good, no good),’ the second may reply. ‘I’ve had a reg’lar troseno (bad sort) to-day. I’ve been doing b—y dab (bad) with my tol (lot, or stock)—ha’n’t made a yennep (penny), s’elp me.’ ‘Why, I’ve cleared a flatch-enorc (half-a-crown) a’ready,’ Master Whelkey will answer, perhaps. ‘But kool the esilop (look at the police); kool him (look at him) Curly! Vom-us! (be off). I’m going to do the tightner (have my dinner).’"
HenryMayhew and John Binny, The Criminal Prisons of London, 1862