Or similarly, “It’s none of your business.” These generally come off as if the speaker is saying that not only does the topic not concern you, but you have no business asking about it either.
These words may come up when one person asks about a situation that is private. In that case, a better way to respond is: “This is their story; it isn’t mine to tell. I would do the same for you.”
Another time these words are said is when someone brings up a topic because they feel that they are left out of an important issue. A “none of your business” response is very demeaning. Obviously, since they have brought it up, they feel that it does concern them. They will likely react strongly to your words by either being more forceful or resentful.
A better way would be: “I’d rather not say.” Or “It’s a personal matter; I’m not comfortable talking about it.” “I’m sorry for not mentioning it before.” Or “It’s complex. I don’t feel ready to talk about it just yet. I still have to sort things out.”