I think I’ve linked to this site before, but it’s worth a second look.
link to slang terms
The slang language of the costermongers is not very remarkable for originality of construction; it possesses no humour: but they boast that it is known only to themselves; it is far beyond the Irish, they say, and puzzles the Jews. The root of the costermonger tongue, so to speak, is to give the words spelt backward, or rather pronounced rudely backward, -for in my present chapter the language has, I believe, been reduced to orthography for the first time. With this backward pronunciation, which is very arbitrary, are mixed words reducible to no rule and seldom referrable to any origin, thus complicating the mystery of this unwritten tongue; while any syllable is added to a proper slang word, at the discretion of the speaker.
Slang is acquired very rapidly, and some costermongers will converse in it by the hour. The women use it sparingly; the girls more than the women; the men more than the girls ; and the boys most of all. The most ignorant of all these classes deal most in slang and boast of their cleverness and proficiency in it.
OED on costermonger:
[f. COSTARD an apple + MONGER dealer, trader.]
a. orig. An apple-seller, a fruiterer; esp. one that sold his fruit in the open street. Hence, b. Now, in London, a man who sells fruit, vegetables, fish, etc. in the street from a barrow.
1514 BARCLAY Cyt. & Uplondyshm. (Percy Soc.) 2 Than [was he] a costermonger.