Criticizing or humiliating someone — usually women — for their real or presumed sexual activity, under a gendered double standard.
The word is newer than the thing. Policing women for sex is ancient; naming that policing as a discrete, criticizable act is a product of the 2000s feminist blogosphere. That naming is the whole point — once "slut-shaming" exists as a word, the reflex to call a woman a slut stops being neutral common sense and becomes a behavior with a name, one you can be caught doing.
The common misconception is that slut-shaming is about actual promiscuity. It isn't. The label rarely tracks anyone's real sex life — it tracks perception and enforcement. A short skirt, a nude that leaked, a rape survivor asked what she was wearing, a teenager sent home over a dress code: none require the target to have done anything. The tell is the double standard. The same behavior that reads as confidence in a man reads as evidence of a defective woman. That asymmetry — not the sex — is the mechanism.
It also isn't only men doing it to women. A lot of slut-shaming is horizontal: women policing other women, because the standard has been internalized as respectability. And it doesn't require the word "slut." It shows up as concern ("she has no self-respect"), as gossip, as a school policy about shoulders and straps, as a courtroom question about a survivor's history. Naming it doesn't make it stop — but it moves the burden of explanation onto the person doing the shaming, which is where it belongs.