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.