Mir is a word for a last line of defence to make sure you end and win that arguement between your best friend or worst enemy. It works 100% of the time 60% of the time.
Josh: Dude you just screwed up on that art procect your gonna get a F now.
Mitch: Nah i added design i'll defineetly get and A.
Josh no trust me your gonna get a F.
Mitch: You know what mir.
The conversation is now over you have left your enemy babbling and trying to find words for a come back.