Sandra Fluke heard it when she talked about insurance coverage for birth control. Sara Brown from Boston told me she was first called it at a pool party in the fifth grade because she was wearing a bikini. Courtney Caldwell in Dallas said she was tagged with it after being sexually assaulted as a freshman in high school.

Many women I asked even said that it was not having sex that inspired a young man to start rumors that they were one.

And this is what is so confounding about the word "slut": it's arguably the most ubiquitous slur used against women, and yet it's nearly impossible to define.

The one thing we do know about "slut" is that it's the last thing a woman should want to be. Society is so concerned over women and girls' potential for promiscuity that we create dress codes, school curricula, even legislation around protecting women's supposed purity. Conservative columnists opine that women having sex is tantamount to a "mental health crisis", and magazine stories wonder if we're raising a generation of "prosti-tots".

Leora Tanenbaum, the author of SLUT! Growing Up Female with a Bad Reputation, told me that "a 'slut' is a girl or woman who deviates from norms of femininity. The 'slut' is not necessarily sexually active – she just doesn't follow the gender script."

This nebulous, unquantifiable quality of the slur is what makes it so distressing – there's no way to disprove something that has no conclusive boundaries to begin with. And because it's meant to be more of an identity than a label, it's a term not easily shaken off. "Slut" sticks to a person in a way that "asshole" never will.

So what makes you a slut? It seems the the only hard and fast rule is that you have to be a woman.

Men, of course, are immune – absent, really – from the frenzy of concern. For instance, a new study out of the University of Michigan showed that teen girls who "sext" are called sluts while boys who do the same remain free-from judgement. In another example, the American Medical Association breathlessly released a study in 2006 with the headline "Sex and Intoxication More Common Among Women on Spring Break", intended to warn women about their "risky" behavior while on break – but there was nothing about the men the majority of these young women would supposedly be having all this drunken sex with.

As always, women are sluts and men are, well, men.

For those who haven't had the pleasure of being called promiscuous, it may be hard to understand just how profound an impact it can have. Women's value and morality are closely – though wrongly – tied to their sexuality. So "slut" (or any of its variations) is an accusation with power behind it.

When multiple attackers videotaped themselves brutally raping an unconscious teen girl in California, for example – stopping to take dance breaks and find new objects to penetrate the young woman with – the first trial resulted in a hung jury because the defense argued she was promiscuous. "The things she wanted done were done", argued one lawyer. Another asked the jury: "Why was her vagina and anus completely shaved? Sex! She's a sexual person!"

The accusation doesn't have to be that explicit to have real power. Cherice Moralez – raped by her 49 year-old teacher when she was just 14 – was called "older than her chronological age" by the judge in the trial – a more diplomatic way of saying she had it coming. Her attacker was sentenced to 30 days in jail. Moralez later took her own life.

Multiple girls have taken their own lives of late after being "slut-shamed" – an indication that the slur shows little sign of waning in the damage it does personally.

Tanenbaum, whose forthcoming book is I Am Not a Slut: Slut-Shaming in the Age of the Internet, said that many of the girls she interviewed "had intentionally embraced the 'slut' label as a badge of honor to advertise their sexual empowerment."

But, she added, "they ended up losing control of the label when their peers turned it against them".

Broader efforts to "reclaim" the word – via marches like SlutWalks, for instance – have largely failed. While the anti-rape protests that spread across the country a few years ago were popular in terms of attendance and media coverage, and I was an early supporter, many women felt the word "slut" was irredeemable - especially women of color, for whom racist stereotypes about their supposed innate promiscuity always presented a unique danger.

The "slut" idea hurts women politically as well. A safe contraceptive and a cancer vaccine were both held up for years because of fears they would make women "slutty", and anti-choice legislators and activists insist that that abortion providers are in the "business" of promiscuity – and use that accusation as a way to defund critical health care providers like Planned Parenthood.

So, what's a "slut", then? It's any of us, and all of us – especially those of us who step out of line in some way real or imaginary. It has little to do with the number of our sexual partners, or the way we dress or flirt, or if we take birth control or not.

It's a warning more than a word – a reminder to women that we must adhere to the narrow standards of femininity and sexuality set out for us, or be punished accordingly. And in that way, the real meaning of "slut" is terrifyingly clear.