To tell something about someone that they are trying to keep a secret, usually in a public format or to someone important, like a relative. Mostly used when saying that someone is part of the LGBTQ+ community, or has some form of kink or hidden lifestyle.
A girl, mad at her friend posts this on social media:
"She's a lesbian and a furry anyways, and into BDSM too."
Another friend replies:
"That's really harsh. You didn't have to out someone like that."