As for where it comes from, most people imagine a mythological fairy that would dance out of the woods wearing a veil of rotting flesh and steal an innocent teenage boy's virginity.
It's the boogeyman an MRA activist will tell his kids about if he ever manages to impregnate someone.
But it used to mean ...
Six hundred years ago, while Geoffrey Chaucer was banging out The Canterbury Tales in the middle of a gambling binge, he used the world "harlot" not to describe the type of lady who enjoys dancing to Def Leppard, but a goofy, somewhat mischievious man, because that's what it meant. Back then harlots were "knaves," "rapscallions," "roustabouts," and other awesome words that you should all start using so it's not just me anymore. So how the hell did this happen?
Turns out we have no one to thank more for our ever-increasing library of synonyms for "slut" than the Bible. In the 15th century, "Harlot" was used to refer to people in the entertainment business, but when the Geneva Bible was published in 1560, the translators had used the word "harlot" in places where the previous Bibles had used "strumpet" or "whore," because the difference between "actor," "prostitute," and "slut" wasn't very important in that context, which can only mean that the Geneva Bible was written by that same drunk grandmother, right after she watched something starring Scarlett Johansson.
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"This scene is disgusting. She should be wearing a bathing suit." -- The Drunk Grandmother Bible