A phrase used to express apathy towards another's disapproval. Probably one of the most overused and annoying phrases of the year 2011 and will be for 2012. Although it has reason behind it in most cases (mainly when it really is only a matter of opinion) in some cases it does not (when reason comes into play).
An acceptable use of the phrase:
Sam: You look weird as a brunette.
Sandy: Haters gonna hate.
An unacceptable use of the phrase:
Sam: Justin Bieber can't sing and looks like a 12 year old girl.