Used specifically in the city or region of Philadelphia to describe a fight or a situation that posses the ability that could escalate into a physical confrontation.
Somewhat of a rhetorical question, where one fellow male asks another if he's admiring the aesthetics of your body definition, or weight lifting ability, when in reality it is actually quite obvious.
Josh finishes a set of dead lifts; with impressive form and weight - when he looks over to his fellow mate, who seems to be struggling with curling 30 pound dung bells. He says " Hey, Steve, U Mirin' Brah? " - Steve replies "No way man", and continues to finish his set.
Josh smirks and flexes in the mirror, and says to himself - " Yeah.. he was definitely Mirin' "
In the music industry, when the "sound guy" makes an opening band sound bad (by poorly mixing them) to make the headlining band sound better.
The practice is mentioned in Five Iron Frenzy's song "Blue Mix" from the album "Electric Boogaloo." Lead singer Reese Roper disclosed the song's meaning in the November/December 2001 issue of HM (Hard Music) Magazine.
In west Philadelphia born and raised
On the playground is where I spent most of my days
Chillin' out maxin' relaxin' all cool
And all shootin' some B-ball outside of the school