A description of Backslang from 'Jottings from Jail' Notes and Papers on Prison Matters 1887 by Rev. J W Horsley, Chaplain of Her Majesty's Prison Clerkenwell.
The second class of manufactured slang is that largely patronised by costermongers. It is called backslang, and simply consists of spelling (more or less accurately) words backwards. Thus "Hi, yob! kool that enif elrig with the nael ekom. Sap her a top o'reeb and a tib of occabot," is only, "Hi, boy! look at that fine girl with the lean moke (donkey). Pass her a pot of beer and a bit of tobacco." The art or merit of this form of slang consists in the rapidity, often most remarkable, with which such words can be reversed. Thus a gentleman wishing to test the skill of a professor of the art with a word not in common use in the market, asked his coster friend what was the backslang for hippopotamus. At once he answered, "Sumatopoppy," the y being euphoniously put for ih.