1.When someone asks you something but you want it to be a secret so you won't say it.

2.Also used when you want to keep an information private.

Got the covid-19 vaccine

Before, it was a phrase to notify someone not to get involved in whatever the situation is or what you're talking about. Now, it basically is apart of poor defense tactics used when a person abruptly joins your conversation, has their say and humiliates you by showing how wrong you are. Commonly used by actors in the show What Would You Do.