Last winter, the World Oral Literature Project at the University of Cambridge released a list of 3,524 dead and dying languages. Among the 150 considered to be in “extremely ­critical ­condition” were Southern Pomo (spoken by Native Americans in California), Gamilaraay (from New South Wales), Mócheno (northern Italy), Istriot (the Croatian coast), and Manx (the Isle of Man). The entry that drew the most attention, perhaps because its speakers are not defined by geography, was Polari, an underground language used by gay men in London until the 1970s.

The existence of a “gay language” is not well known, even in the U.K. A poll of British gay men in 2000 revealed that half of respondents had never heard of it. If Polari is known outside of England, it is most likely because Morrissey once titled an album Bona Drag, which means “nice outfit.” And there is a brief Polari scene—with subtitles—in the 1998 film Velvet Goldmine (“A tart, my dears, a tart in gildy clobber!”). But you’d have to scour a lot of pubs to find anyone who still uses it in conversation. When the Cambridge list came out, Paul Baker, the leading Polari scholar, was surprised that it had been considered endangered, not extinct.

And yet the news of Polari’s demise contained a deeper question: is it a language at all? Most speakers would have considered it no more than a lexicon, designed to trip up outsiders and amuse insiders. As Morrissey sings in “Piccadilly Palare,” from Bona Drag: “The Piccadilly Palare was just a silly slang / Between me and the boys in my gang. / ‘So bona to vada, oh you, / Your lovely eek and your lovely riah.’ / We plied an ancient trade / Where we threw all life’s instructions away.”

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Playful as it is, Polari grew out of punishing circumstances. When it first gained popularity in London, in the mid-twentieth century, homosexuality was a crime that the police were eager to prosecute, and blackmailing was rampant. In a famously tragic case, in 1952 Alan Turing, the cryptanalyst who cracked the Germans’ Enigma code, was caught having an affair with a man. After being tried for gross indecency, he agreed to a “treatment” of female-hormone injections, then committed suicide in 1954.

Given these appalling conditions, embattled homosexuals needed a means to communicate. As Baker writes in Polari—The Lost Language of Gay Men (2002), Polari was “a way of tearing down ‘heterosexual’ reality, and remoulding it as a world seen through gay eyes, with gay standards.” Its users tended to be flamboyant working-class “queens”—arch eccentrics in the model...