1. stands for "must be nice" and used sarcastically when someone tells you something that makes them sounds wealthy, carefree, or conceited.
Wealthy Chick: My parents, my sister and I are going to Aruba for Christmas for a week then to Argentina and Brazil to spend the New Year.
Poorer Friend: MBN! shit!!!
Wealthy Chick: What does 'mbn" mean?
Poorer Friend: it means that it Must Be Nice, mutherfucka! Tell your parents to hook ME up!
must be nice..........
GuyA: Damn I'm full, huge steak dinner.
GuyB: I'm eating chicken Ramen, mbn.