Not letting someone join in on your conversation or you don't want to tell another person what you are talking about.

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.

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.

a wicked ass chick on myspace, she may be none of your business but hell every guy and girl wants her to be their business!

when you talk to your therapist about your friends being suicidal and then they get put into a mental hospital. when it’s none of your business

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 lame way of saying I don't want you to know what I am talking about.