Around the time Steve Bannon lost his spot in President Trump’s National Security Council, a reporter at the Daily Beast spoke to an unnamed White House official who described Bannon’s rage. America’s most powerful alt-righter had “vented to us about Jared being a ‘globalist’ and a ‘cuck,’ ” the official said. “He actually said ‘cuck,’ as in ‘cuckservative.’ ” And there it was, the moment when a niche neologism moved, fully and incontrovertibly, into the mainstream — and explained the neurotically masculinist world whence it came.

Cuckservative, the portmanteau that insults moderate Republicans by accusing them of being cuckolds — men whose wives cheat on them — started trending in the alt-right two years ago. The day after Donald Trump announced his candidacy, a thread on 4chan’s /pol/ page titled “Jeb Bush the cuck” featured a photo of Trump yelling, “You’re fired,” alongside a slew of sexually and racially charged insults. The metaphor gained momentum when, one month later, Rush Limbaugh marveled at Trump’s refusal to become an “average, ordinary, cuckolded Republican.”

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The first recorded use of cuckold is in a 13th-century Middle English poem called “The Owl and the Nightingale,” in which two birds debate gender relations. If a man mistreats his wife, the (sort of feminist) owl argues, “if she cuckolds him, God knows, it’s not her fault.” The term derived from cuckoo, a bird known for laying its eggs in other birds’ nests. Chaucer, Spenser, and Shakespeare all made frequent use of the term. In The Canterbury Tales, Chaucer tells the story of an elderly cuckold named January who is blind. Even after the gods briefly restore January’s vision so he can see his wife copulating with another man in a pear tree, she convinces him that he is mistaken and continues cuckolding him.

So how did a literally medieval insult end up in modern American politics? Some blame pornography — in the past decade, the archaic term has enjoyed a resurgence in online smut and fetish sex, where it functions as an emasculating subgenre of partner-swapping. As a modern sexual practice, cucking requires an ostensibly heterosexual man to watch as another man penetrates his wife. The goal, generally, is eroticized humiliation of the cuck as a form of emotional masochism. (Leopold von Sacher-Masoch, after whom masochism was named, was a fan.) In 2016, PornHub analyzed cuck fetishists’ searches and found that “humiliation” and “bisexual” were among the top words to appear in “cuckold” searches.

And, yes, race also plays a role. “Interracial” was the sixth-most-common term to appear next to “cuckold” in searches, and race plays a role in a significant portion of the cuck-porn oeuvre. Cuckoldry is, after all, about power — and when Americans talk about power, they talk about race. So it’s not surprising (but still highly upsetting) that humiliation fantasies that hinge on female sexual autonomy would over-index with those who view black sexuality as threatening to their power. Historically, also, cuckoldry incorporated race anxiety, such as the fear of losing white women to Othello in Shakespeare’s play. Still, PornHub users are more likely to pair “cuckold” with “husband,” “amateur,” and “cleanup.” (It’s a cum-eating thing.)

As for cuck’s leap from porn to politics, that’s sort of anyone’s guess — but it probably has something to do with the internet, too. Politico cites a political-message-board uptick that dates to Gamergate, the social-media furor that revolved around a female video-game designer whose ex-boyfriend accused her of cheating. When anti-feminist activists moved from video games to politics, Politico’s Ben Schreckinger argues, they brought their lexicons (and chauvinism) with them.

For the political figure most associated with cuck-phobia to be Donald Trump is either darkly appropriate or painfully ironic. Trump does seem to subscribe to a worldview in which power is tied to one’s ability to obtain — and guard — sexual partners. And he’s on the record plotting to cuckold other men, most notably in that leaked Access Hollywood tape. But he’s also a man whose wife maintains more corporeal autonomy than any previous First Lady — Melania spends most nights hundreds of miles away from Donald, and even on the nights they’re together, the couple are rumored to use separate beds. (Melania’s team denies this. Traditionally, however, “wearing the horns” was as much about perception as reality.) Meanwhile, Trump has associated himself with people so chaste that, according to medieval cuckold theorists, they’re destined to wear the horns, too. In The Decameron, 14th-century Italian writer Giovanni Boccaccio describes a man whose piousness drove his wife to run off with a pirate. What I wouldn’t give to find out how Boccaccio would deal with a vice-president who thinks extramarital dining is a gateway to sin.

But the greatest irony is how self-defeating cuck is as an emasculating insult. For a man to use cuck is to admit that his entire understanding of masculinity revolves around the actions of women — which turns masculinity into a quality that women, not men, control. Just as a woman who uses bitch to insult other women in earnest is, inevitably, also kind of a bitch, a man who accuses another of being a cuck is, inevitably, the most fragile man of all.

*A version of this article appears in the May 1, 2017, issue of New York Magazine.