A rude way to prevent someone from joining a conversation when they’ve done absolutely no wrong
Person 1: what kind of music are you into?
Person 2: I like pop music, especially J-pop, that’s the best kind
Person 3: I like J-pop, too!
Person 2 to Person 3: that’s cool
Person 1 to Person 3: None of your business, GTFO
Person 3 leaves and cries
Stay out of someone's conversation keep information that you want kept to yourself that you do you not want getting out there. A pretty lameway of saying I don't want you to know what I am talking about.
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.
Boyfriend: Now get back inside, try on some clothes that makes you look more hot. NOW!!
Old Woman: Hey! Your girlfriend can wear what she likes! Your just being rude!!!
Boyfriend: Excuse me, let me sort this. It's none of your business.