Last weekend, a friend of mine was sitting on a park bench when she felt a presence sneaking up from behind – and noticed an older man taking pictures of her exposed back. When she told him to stop, he yelled back at her that it was his "First Amendment right to be a creep". Little did either of them know that he was articulating the foundation of new legal battle about the internet and, well, the right to be a creep.

The US supreme court announced on Monday that it will hear arguments in a case – Elonis v United States – about whether threats made on social media are protected by free speech. It is a watershed moment for anyone like me who believes that online harassment is often scarier than in-life harassment. When someone catcalls you on the street or says something threatening, you can use your best judgment to ascertain how dangerous he is. When you're threatened online, you have no way of knowing – and that lack of context is terrifying.

And Anthony Elonis, the plaintiff in the supreme court case, seems like a pretty terrifying man. He was sent to prison after posting harassing and violent threats about his wife and co-workers on Facebook. One such post read:

There's one way to love you but a thousand ways to kill you. I'm not going to rest until your body is a mess, soaked in blood and dying from all the little cuts. Hurry up and die, bitch, so I can bust this nut all over your corpse from atop your shallow grave.

Elonis's wife, understandably afraid, received an order of protection that subsequently led to more threatening posts from him – including one about slitting the throat of the female FBI agent who had been dispatched to his house as part of the case. Apparently anticipating the charges he could have been facing, Elonis wrote on Facebook that the posts were meant to be "therapeutic" and not threatening – and cited his free speech rights.

Despite his protestations, Elonis was arrested and convicted on federal charges of using interstate communications to convey threats, served more than three years in prison and is currently out on supervised release.

The issue for the high court now is what constitutes a "threat": whether it's enough for a person like Elonis's wife to be reasonably afraid given the threatening posts, or whether a jury should determine what someone like Elonis's actual motivations were – and whether the social media context of such threats makes it clear that the words are unserious.

The courts have upheld that you can't yell Fire! in a crowded theater and claim the protections of the First Amendment to avoid consequences. Now they'll have to decide whether you can post I'm going to rape you on Twitter, or I'm going to kill you on Facebook and claim it's your constitutional prerogative.

Dahlia Lithwick at Slate wrote that "one of the confounding factors here is that the court hasn't yet looked at the question of true threats through the lens of modern technology."

Those urging the court to take the case argued that speeches at rallies and even cross burnings are fundamentally different from YouTube postings or tweets. Elonis claims that you can't use an objective listener standard when you are dealing with the interpersonal and context-free conversation that takes place in the Wild West of social media.

Basically, Elonis and his lawyers want us to believe that if threats are on social media, they can't be treated like real life threats – even if those threats make a reasonable person very afraid.

But University of Maryland law professor Danielle Citron cited a study in her upcoming book (for which I was interviewed), Hate Crimes in Cyberspace, showing that when undergraduate students were polled about the impact of different kinds of sexual harassment, the kinds that happened online were rated as the most distressing precisely because "the absence of cues about attackers led participants to fear the worst".

Of the Elonis case, Citron told me:

There's no way to hear if there's laughter in his voice, for example. But we know he's angry, he's been fired from his job, he's been known to sexually harass women. When we don't have physical cues, it makes the threat more frightening.

The battle to recognize the legitimacy of harassment that happens online has been going on for years. But as much as some people would like to claim that social media is an inherently unserious place where "haters gonna hate", several recent tragedies tell a different story. In a post-Isla Vista world, we need to err on the side of caution and take threatening speech for what it is – a threat – and believe it.

In 2007, Kathy Sierra – a game developer and well-known software programmer – started to receive threats from anonymous harassers. When she spoke out, the harassment got worse. She received Photoshopped pictures with nooses next to her head, she was threatened with rape and suffocation, and people posted her home address and social security number in online forums. Terrified, Sierra cancelled all of her speaking engagements and said she would retreat from public life online.

Sierra's case was a landmark moment for women on the web – it marked the beginning of a growing public acknowledgment that online harassment is serious, and a serious problem. Her defenders, myself included, argued that online threats were problematic and terrifying. At the time, however, others thought that they were just the price of doing business on the internet. Markos Moulitsas, a then-prominent liberal blogger and the founder of Daily Kos, infamously wrote that if those being harassed "can't handle a little heat in their email inbox, they should try another line of work".

Ever since, we've seen an uptick in social awareness about social media harassment and the way technology is used to terrorize people – especially women and marginalized communities. Now everybody knows what "cyberbullying" and "slut-shaming" are, and there's even the occasional mainstream frenzy around such cases. But there's been little done in the way of policy and the letter of the law.

When online harassment and threats against me reached peak levels some years back, well after the Sierra case, I reported it to the authorities. But there was little law enforcement could do, so they advised me to leave my home, change my routine and watch out for people or cars that I saw more than once in my neighborhood. So my husband, baby and I left our home and I began changing my life in ways big and small to accommodate this new reality of harassment. I shouldn't have had to – and neither should anyone else.

Citron told me part of the problem is that the laws we do have were written before the internet was around, and part of what the supreme court may be doing is catching up with history. She also told me that people still argue - archaically, in her view - that laws against online harassment are akin to "criminalizing speech" and that "you shouldn't legislate personal messiness".

"But", she added, "in the 1970s we decided that domestic violence wasn't just interpersonal messiness, and that we were going to come in and say No, you can't beat your wife."

And that's the thing: we don't give up on legislating wrong-doing because it's messy or personal. No one is arguing that people can't say hateful things or call people names; the emails I get calling me a "slut" and a "feminazi" are immoral, but they're not (and shouldn't be) illegal. Threats and the invasion of privacy, though, are different – even in the most public of places: your right to be a "creep" doesn't trump everyone else's right to a life free from violence and the fear of it.