Abbreviation for "Barack is in the White House"
A quick way one person (usually black but not limited to black people alone) tells another (again usually black but not limited to black people alone) to behave in a respectable manner or they'll get smacked for acting the fool.
Beavis: I am cornholio!! I need TV for my bung-hole!!!
Butthead: Dude BIITWH! Don't make me smack you fool
John: <Making a lot of noise while Jesse Ventura is speaking on Fox News>
Wayne: Dude BIITWH!! Let me watch Jesse Ventura lay into these fools!!