What we now regard as the first glossaries of English slang—they were actually collections of criminal jargon—appeared in the 1500s. Looking back, some of the details may be blurred, but the themes that would characterize slang for the next half a millennium are already evident. There’s sex (to wap), intoxication (booze), and money (lour); man (cove) and woman (mort); the penis (jockum) and the vagina (cony). There’s buttocks (prat) and excrement (turd), thieves (prigs) and policemen (harman-becks). And there’s stupid (blockish) and mad (lightheaded).

When I consider the 125,000 slang words and phrases I’ve collected over my career, from five continents, filling an ever-expanding database, one thing’s clear: slang is repetitive. It is touted as speech’s cutting edge, yet its preoccupations are unchanging. The jury may be out on whether slang is a language—it’s got no grammar, which means it isn’t; it’s an undeniable form of communication (perhaps the liveliest), which means it is—but either way, it is an unrivaled repository of synonyms.

Slang’s obsessions embody what Freud called the id and I call humanity at its most human. Where Standard English cheerfully makes do with single terms—sexual intercourse, drunk, stupid—slang offers respectively 1,750, 2,500, and 1,000 variations. The same goes for slang’s other chart-toppers: we find myriad entries in such categories as drugs, criminals, police officers, the genitals, money, and so on. Why? Why do we need so many variations on these themes? Why 1,200 penises, and 1,200 prostitutes? Why, for Christ’s sake, 220 ways of saying vomit? Enough, surely, should be enough, even for the most obsessively foulmouthed.

Sorry. That isn’t the way slang works. Slang is not about word count, but about in crowds and outsiders, and self-definition. We are we because we know the words; you are you because you don’t. Whereas Standard English speaks for authority, slang gives voice to the margins. Its original in crowd was criminal. The cops, who didn’t know the words, were excluded. Modern slang’s great constituency is the young; parents and teachers have replaced cops as the outsiders.

Which brings us to why slang words replace themselves so regularly. If one person has a secret, another will betray it. And when a piece of slang escapes into the wider world, it leaves a gap that must be filled. So while the slang of the 16th century has mainly vanished, its descendants march on. We lose wap and get bumbaste, lose that and get trounce, lose that and get strum. And on it goes, until we have 1,750 terms for sex.

You might expect this lineage to die off. In an era of surveillance and social media, of confessionalism and dwindling taboos, why bother generating secret new words for old preoccupations? And yet take a look at the latest batch of slang I’ve compiled. Multicultural London English, as academics call it, blends elements of American rap, British grime music, Jamaican patois, and London Cockney. A vocabulary that cuts across class and color to an unprecedented extent, it’s definitely new. Or is it? Some examples: gash (women), shotting (drug-dealing), wonga (money), merk (murder), lash (intercourse). Here we go again.