Move over, Webster’s—Elon Musk apparently runs the dictionary now. And wouldn’t you know it: Musk’s dictionary is weird and transphobic.

The Twitter owner issued a decree on Wednesday in one of his favorite places—the replies to someone else’s tweets. Musk announced that going forward, “the words ‘cis’ or ‘cisgender’ are considered slurs on this platform,” and that repeated use of the terms would lead to temporary suspensions at a minimum.

For the purposes of removing all doubt: “Cisgender” is not, by any commonly understood definition of the word, a slur. It’s a term adopted by the medical community to describe someone who aligns with their biological sex and gender from birth (“cis” is a Latin prefix meaning “this side”), as opposed to transgender people, who do not.

In a clinical sense, the word “cisgender” is a way for medical practitioners to differentiate between a transgender patient and someone who identifies as their birth sex or gender without having to label them as “normal” or a control group.

So how did a term that has been largely embraced by the medical community wind up becoming an Internet-poisoned slur to the owner of one of the world’s largest social networks? To answer that question, we have to follow two strands. One involves the long and rich legacy of transgender health care. The other involves Elon Musk’s decision over the past several months to turn Twitter into a haven for people who share his increasingly fanatical anti-trans ideology.

Dr. Dana Defosse, a retired researcher, said that she had modest goals when, as she claims, she coined the term “cisgender” on an old Usenet newsgroup where transgender people connected in the early 1990s.

“I knew that in chemistry, molecules with atoms grouped on the same side are labeled with the Latin prefix ‘cis–,’ while molecules with atoms grouped on opposite sides are referred to as ‘trans–.’” So, cisgender. It seemed like a no-brainer. I had no idea that hitting ‘enter’ on that post would start an etymological time bomb ticking,” Defosse wrote in a HuffPost op-ed about the origin of the word earlier this year.

Defosses’s claim is not universally accepted; for instance, Carl Buijs, a Dutch transgender man, said on an old Google discussion group that he coined the term independently.

Regardless of who was the first to use the term cisgender online, there exists evidence from the mid-1910s that German sexologists used similar terms to clinically describe patients, showing just how far back the field of transgender medicine does in fact go.

Still, the word only recently gained currency—it was added to the Oxford Dictionary just eight years ago, in 2015. But that was the perfect time for this benign scientific term to run straight into the growing wave of transphobia brewing online.

In the mid-2010s, a growing number of cis women began insisting that trans people posed a threat to their “sex-based rights.” These “trans-exclusionary radical feminists” (TERFs), or “gender criticals,” as they would come to label themselves—posited that their being labeled as “cisgender” without their permission was tantamount to their being called a slur, as they did not recognize trans people’s existence.

According to the website TransAdvocate, anti-trans posters started pushing the idea in 2013 that the words “cis” and “TERF”were misogynistic slurs aimed at silencing a supposedly marginalized community.

This was initially perceived as somewhat niche Internet drama, largely taking place on Twitter. Since then, though, the anti-trans movement has found allies in celebrities such as J.K. Rowling (who shared a warm exchange with Musk after his anti-“cis” ruling), and a political home in both the Republican Party and the ruling Conservative Party in the United Kingdom. Consequently, anti-trans rhetoric has become far more radicalized.

For instance, many online commentators have rushed to push the notion that the term “cisgender” was created by German sexologist Volkmar Sigusch. Sigusch was the director of the Institut für Sexualwissenschaft (Institute for Sexual Science) at the clinic of Goethe University in Frankfurt from 1973 to 2006. One of his publications from the 1990s, “Die Transsexuellen und unser nosomorpher Blick“ (“Transsexuals and our nosomorphic view”), contained the word “cissexual” as an antonym for “transsexual.” Online anti-trans activists have seized on some controversial statements in a 2011 interview in Der Spiegel to depict him as a “pedophilia apologist,” thus poisoning the well for the term cisgender forever.

This is where Elon Musk comes in.

Since he bought Twitter last year, Musk has used the platform to send a series of ever-more-blatant messages that he has fully bought into anti-trans hysteria. He has repeatedly shared transphobic content and made transphobic comments, peddled false far-right talking points about trans health care (at one point suggesting that doctors who care for trans children should “go to prison for life), enthusiastically cheered on transphobes using his service, and—perhaps most troubling of all—loosened protections against hate speech toward transgender users. (Not surprisingly, Musk’s trans daughter has cut ties with him, a decision Musk attributed to the influence of “neo-Marxists.”)

It was only a matter of time, then, before the campaign against the term “cisgender” found a home with Musk.

Musk’s increasingly vicious transphobia has not happened in a vacuum. It is part of a broader conspiracy to connect being transgender to pedophilia and child grooming. As more and more on the political right continue to attempt to brand the affirmation of young peoples’ transgender identity as “child abuse,” the more they have begun to weaponize the powers of the state to investigate families providing affirming care for their kids.

Moreover, Twitter is not just a social media platform. It has been called the assignment editor of the right-wing media ecosystem, especially during and after the Trump administration. Entire media careers have been made out of demeaning trans people online, thanks to the platform’s Musk-era extremism. And Twitter is still a major organizing hub for trans people.

This is why it matters so much that Musk has gone down this path. Now, by unilaterally deciding that “cisgender” is a slur—and thus sending yet another message about supposed extremist bullies in the pro-trans camp—Musk is once again declaring open season on trans Twitter users. For trans people, this will mean more targeted harassment, and possibly less protection against their harassers. Many anti-trans groups have for years bombarded trans people with targeted harassment campaigns, and it is reasonable to assume that, given the clear stamp of approval from the person in charge of Twitter, these groups will feel emboldened to escalate their tactics, all while trans people face retribution for asserting their identities online.

That is a terrifying thought to anyone who just wants to exist as a trans person online—and anyone seeking to figure out who Elon Musk has become needs to understand that whipping up hatred against trans people is now a core part of his mission.