Of all types of slang, perhaps the best known is Cockney rhyming slang. It developed in London during the first half of the 19th century, at about the same time as Robert Peel was inaugurating his police force, perhaps to be used by criminals as a secret language that the slops (back slang, from ecilop) would not understand. Later, it spread with the convicts who were deported to Australia, and then to America, where it became known as Australian slang.
Several now familiar words in the English language had their origins in rhyming slang. For example, a dicky is a shirt because it is the shortened form of “dicky dirt,” and perhaps there is an association with the German Decke, a cover. “Dickey” on the other hand means unwell, from “Uncle Dick,” sick. To rabbit (rabbit and pork) is to talk. The brass tacks that people get down to are facts. The rude noise known as a raspberry is actually a raspberry tart. And who among us would call someone a silly berk if they knew that the word originates from rhyming slang “Berkshire Hunt”?
The best rhyming slang uses expressions that themselves reflect the meaning of the original word. For example, of the several expressions for syphilis, the best is bang and biff, which not only rhymes with “syph” but also carries a suggestion of how you might catch it. Then there is a “snake's hiss,” meaning an act of urination (oh, all right, a piss), which conveys its meaning through the sound that it suggests, not to mention the suggestive shape of the snake. Occasionally the meaning is subverted, as in “do me good”, a Wood (Woodbine cigarette). Humour is important too. For instance, the Jim-brits or Jimmy Britts, shortened to “the jimmies,” is Australian rhyming slang for diarrhoea; “Jimmy” (or “Jimmy Grant”) is an immigrant, so not only is this a deft expression, it is also a neat insult of the Australians' traditional enemy.
So the next time you're feeling a bit Tom, Harry, and Dick or get frock and frill with a bout of threepenny bits or a dose of the horse and trap, and you think you're going to crack-a-cry, go to see the King's Proctor and ask him for some Jenny Hills to make you feel all harbour light. Under-cum-stumble?