Slang terms, almost by definition, don’t bear straightforward connections to the concepts they describe—what is “on fleek” and why is it a desirable quality?—but in the Cockney case the chain from the slang term to the object it describes can be especially convoluted. It’s relatively simple to trace how “China plate” becomes “mate,” but sometimes you have to go through several more steps, including dropped rhymes, half-words, and slang terms for other slang terms, to get anything resembling the original idea.

Witness “ala,” for “posterior,” the personal favorite of Jonathon Green, a London-based lexicographer and the author of a three-volume dictionary of slang. Ala is short for alabaster. Alabaster is a material related to plaster of Paris. Paris rhymes with Aris, which is short for Aristotle. Aristotle rhymes with bottle. Bottle is short for bottle and glass, which rhymes with ass or arse (itself a slang term). “Finally,” Green told me, “after this long and complex, and possibly absurd, road that we’ve traveled, we get to the standard English ‘buttocks.’”

Whimsical, yes. But there are rules, or at least there were in the early days of the form in the 19th century. Technically it’s not “rhyming slang” if you substitute “giraffe” for “laugh” just because the first word rhymes with the second, said Green. The rhyme should come from a phrase or compound, or a full name—“bottle and stopper” for “copper” in the sense of a policeman, or “Germaine Greer,” the feminist scholar, for “beer.” A true pro would obscure the meaning further by dropping the part of the phrase that actually rhymes, thus: “I’ll take a pint of Germaine, and one for my China here.”

When John Camden Hotten—whom Green described in his book Cassell’s Rhyming Slang as “a bookseller, pornographer, and slang lexicographer”— first documented the phenomenon of rhyming slang in his 1859 Dictionary of Modern Slang, Cant, and Vulgar Words, these rules basically held. (Green attributes to that book the first appearance of the phrase “rhyming slang,” but it also serves as a guide to “long and windy slang words” like “slantingdicular” as well as “parliamentary slang” and “military and dandy slang.”) The form had a limited vocabulary of perhaps a hundred examples at the time; Green estimates it’s now grown to more than 3,000 phrases.

As for why anyone would bother to choose from, by Green’s count, 18 different inscrutable terms for tea (“Mother McCree,” “sailors on the sea,” “split pea,” etc.), rather than just saying tea, there are a few different versions of the rhyming-slang creation myth—including the notion that it was invented by criminals using a form of verbal encryption to confound the police. “The most popular idea,” Green said, “is from East End market traders known as costermongers—from costard apples, but they sold a lot more than costard apples. ... Given that rhyming slang is so intertwined with the Cockney image, the London East End image, I’m sure that that’s the right one. But the interesting thing is that you actually had a moment of creation. Most slang just comes out of nowhere.”