English slang is a field rich for tillage, as Max Décharné proves in “Vulgar Tongues,” a triumph of philological research and mordant social commentary. To grok it righteously, though, we must abandon any notion that slang terms are somehow new.

Acting fly, for example. Fairly recent coinage, right? Vigorous head shake from Décharné: The word was current in London 200 years ago, when lexicographer Francis Grose — the author’s hero and inspiration — defined it as “Knowing. Acquainted with another’s meaning or proceeding.”

Or perhaps you think it’s hip to flaunt your geek cred, unaware that the word appeared in Shakespeare’s “Cymbeline” (though with a geeky final “e” appended).

Even rap has been kicking around for more than two centuries. It showed up (as rap out, along with hip-hop and punk ) in Samuel Johnson’s 1755 “Dictionary of the English Language” — which defined it with startling contemporaneity as “To utter with hasty violence.”

Décharné, a British music critic and language freak who moonlights as the keyboardist for the Flaming Stars, has wolfed down encyclopedic gobbets of American pop culture. In 2000, spurred by “a lifetime’s obsession with the language of vintage pulp crime fiction, film noir and jazz, blues, hillbilly, rockabilly and rock ’n’ roll music,” he published “Straight from the Fridge, Dad: A Dictionary of Hipster Slang.” He broadens that ambit ambitiously in “Vulgar Tongues,” which aims (both high and low) to tell the story of how “the English language of Shakespeare’s day fragmented and twisted into all kinds of shapes, as people like pickpockets, beggars, sailors, musicians, gangsters, whores, politicians, gypsies, soldiers, gays and lesbians, policemen, rappers, cockneys, biker gangs and circus folk seized the King’s or Queen’s English by the throat and took it to places it would probably regret in the morning.”

That facetious bill-of-fare deflects the intense scholarship that Décharné has poured into “Vulgar Tongues,” whose bibliography cites more than 500 books and periodicals (but whose index sinfully omits individual slang terms). We watch hipster devolve from a “proud flag of suavedom” in the 1940s bop era to a dismissive insult in 2017. (At least it didn’t sink from cool to embarrassing at the speed of twerk.) We learn that 76 years before Simon and Garfunkel sang about “feeling groovy,” the term denoted someone “stuck in a groove, or a rut . . . settled in habit; limited in mind.” And we look over Herb Caen’s shoulder as he coins beatnik in his San Francisco Chronicle column of April 2, 1958, borrowing the root from “the beat generation” and the suffix from the USSR’s recently launched Sputnik satellite. (To Caen’s everlasting astonishment, the word gained traction instantly; he opened the April 3 edition of the paper to find a headline about a “beatnik murder.”)

Despite such rampant granularity, “Vulgar Tongues” never loses sight of slang’s deeper cultural role: Argot can veil a speaker’s intent, define a group’s identity or enable humans to voice the unspeakable, “from the scurrilous and the obscene all the way through to the shocking and the tragic.” But as Décharné froths himself up about digital monitoring and other modern threats to free speech — notably the “new wave of puritanism [that] has emerged . . . under the cloak of political correctness” — he wobbles offstage on a disquieting note: “Ultimately, slang will have no place in this world, because the best of it is almost guaranteed to offend someone, somewhere.”

To which I can only say, “Right on, bro — I’m acquainted with your meaning or proceeding.”

Allan Fallow is a writer and book editor in Alexandria, Va.

By Max Décharné

Pegasus. 388 pp. $26.95