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3.
MDB
Mega Douchebag -- when someone goes beyond just being the regular douchebag they normally are.
Brian from DcEvo: I was street racing last night w/ my kids in tha car fo sho!!!

Everyone else: What a MDB.
by uh, no July 10, 2008
 
1.
MDB
abbreviation for Masters in Douche-Baggery. Title is granted, normally in an honorary fashion, to an individual who displays a pompous attitude or pretends to know everything about everything. Also added to the end of a name, similar to other titles.
John was clearly using his MDB while working with a client the other day.

Just another lame move by John Q Public, MDB.
by projectaristotle July 16, 2009
 
2.
MDB - Short term for Manic Depression Bipolar.

This is a long-term or a short term Illness, it can be treated by counciling, or medication.

Doctor: You have a common Illness named, MDB.

Person: Oh, Great!
Doctor: You have a common Illness named, MDB.

Person: Oh, Great!
by Mr Psychotic April 26, 2010
 
4.
MDB
Sexual term meaning to take a Mountain Dew Bath with someone. A fantasy to fill a tub with mountain dew(or another favorite beverage) and then share it with a partner.
That chick looks like she just got out of an MDB.
by Jacob Tubbs May 26, 2008