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1.
Worse than a brain fart, a brain shart has far worse consequences. A brain shart is doing something completely idiotic but in a key situation, or in a way that brings about serious repercussions.
Who can ever forget Chris Webber's brain shart in the 1993 NCAA championship game in which he called a timeout when his team had none, thereby getting a techinical foul and giving UNC the crown.

Because of a colossal brain shart on air in front of millions, Imus is out of a job and out on his can.
by Mavericklax4 April 27, 2007
 
2.
Similar to a brain fart, only something nonsensical comes out of your mouth during the thought process (aka a brain fart where a little comes out).
Guy 1(in deep thought): Yeep.
Guy 2: What?
Guy 1: Huh? Oh, sorry... I had a brain shart.
by Joshua Sweeney April 24, 2006
 
3.
Similar to the brainfart, it is the inability to even remember the slightest bit of what you were talking about to the point where someone wonders if you *actually* shit your pants while thinking.
When describing his dog, Rufus had a brainshart and immediately went into a 20 year coma.
by Taterbutthole December 27, 2009