Definition
It's a matter (especially a private or personal one) that you are not entitled to be involved in or informed about.
See Also
How People Actually Use It
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.
"A:Hey dude, where are you from? B:It's none of your business."
Source: Urban Dictionary