If there's one thing bullies can't stand, it's being mocked. Humour is the tool anyone can wield, a shiv that can be hidden on the smallest body.

It's the reason dictators throw cartoonists and comedians in jail.

So it's no surprise that the thing that made Donald Trump lash out and label Hillary Clinton "such a nasty woman" was a joke she made at his expense.

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The incident, which has been taken up as a feminist slogan and has spawned a mall's worth of T-shirts, came at the end of the third presidential debate in Las Vegas on Wednesday.

Debate moderator Chris Wallace had asked a straightforward question about social security funding, and Hillary Clinton began answering in a straightforward way: "Well, Chris, I am on the record as saying we need to put more money into a social security trust fund. That is part of my commitment to raise taxes on the wealthy. My social security payroll contribution will go up, as will Donald's." Then, without pausing for a second, she launched a barb with surgical precision: "Assuming he can't figure out how to get out of it."

Mr. Trump, reinforcing his tendency to lunge for even the most toxic and suspicious-smelling bait, leaned into the microphone. "Such a nasty woman," he said, wagging his finger.

Thud! That was the sound of a million jaws hitting the floor.

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Countless women watching the debate, and many, many men, wondered if they had hallucinated the remark. Hadn't Mr. Trump just said, to a burst of laughter in the auditorium, that no one respected women more than he did? (This is exactly the line that the lion uses on the gazelle, but that's a joke for another time.) Hemorrhaging support among female voters, Mr. Trump should have been making the case that there was room for them on the Republican bus, but instead he slammed the doors shut – while writing "No gurlz allowed" on the outside.

Social media responded with a giant, wondrous gust of mockery. Within minutes, I had friends sending me links to "nasty woman" T-shirts (all proceeds to Planned Parenthood). Everyone thought immediately of Janet Jackson's 1986 hit Nasty, and some clever soul quickly produced a mash-up of Janet in her all mid-eighties glory dancing alongside Mr. Trump.

" 'Nasty woman' is either a new low in electoral discourse or Rihanna's next single," journalist Ronan Farrow wrote on Twitter. The memes piled up as satirists piled on. Humour is one of the most effective ways to respond to bigotry. It exposes the bigot's soft underbelly; hatred and self-importance are targets anyone can hit. This is true even for Hillary Clinton, who usually tells jokes with the awkwardness of a statistics prof trying to keep a class of undergraduates awake.

There is something particularly dangerous about a woman armed with a joke – you only have to look at some of the sexist blowback female comedians get on social media. Or you could look at what happened to Amy Schumer during her stand-up routine in Tampa this week, when she riffed on her support for Ms. Clinton, and her dislike for Mr. Trump. Some audience members began to boo, and others walked out. Two days later, she turned the mockery up to 11 during a gig in New York: "Dearest Tampa, I'm sorry you didn't want me, a comedian who talks about what she believes in, to mention the biggest thing going on in our country right now."

A popular (if apocryphal) quote by Margaret Atwood goes something like this: Men are afraid women will laugh at them, and women are afraid men will kill them. This week I appeared alongside Ms. Atwood at a fundraising breakfast for the feminist advocacy group LEAF. In her keynote address, which was very funny, Ms. Atwood talked about the staggering levels of sexism at work in the American presidential election, and how she had never intended her 1985 novel The Handmaid's Tale to be a work of prophecy.

"We've had a resurgence of misogyny not seen since the witch trials," she said. She wondered about the "witch and devil imagery" aimed at Ms. Clinton, and added, "Maybe we should rename her Hillary of Arc." What else do we call witches? Nasty, of course.

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I'm pretty sure that "nasty woman" will be one of the more popular costumes this Halloween (replacing the actually frightening "sexy Ken Bone"). Saturday Night Live will have fun with it. The gender gap – the percentage of female voters favouring Ms. Clinton over Mr. Trump – which is currently somewhere between 6 and 33 per cent, according to poll trackers fivethirtyeight.com, will likely continue to grow. All this is good for a chuckle or six.

That's one small bright spot in months of bleakness. If only this election were funny most of the time, instead of depressing. If only the first female candidate for a major U.S. party weren't being threatened with incarceration and violence. If only American women's reproductive rights weren't under threat. If only a sizable proportion of the electorate didn't think it was okay for an alleged groper to inhabit the Oval Office. Laugh? I could cry.