To spill out any thoughtless, boorish, insensitive, indiscrete, or brainless crap that comes to mind without being able to exert any control, restraint, forethought or prudence. To talk out of one's ass.
Sorry for what I said about your wife's booty at the meeting this morning, man. I don't know what I was thinking. I must have had an episode of mental diarrhea.
by SirOreo December 27, 2014
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Randomly spitting out too much information that's either relevant or irrelevant to the conversation
Person 1: "Do you know how antibiotics screw with your stomach because our normal gut flora lines our GI tract which protects us from other pathogens. So what the antibioitcs are doing is killing good bacteria in your stomach allowing bad bacteria to colonize. That is why most ppl who have been on antibiotics for too long get explosive diarrhea or clostridium difficile."

Person 2: Mental Diarrhea...
by PeaceOut123 October 24, 2011
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Mental diarrhea is a state of emotional situation, when the effected person is acting with all the emotions at once. This can be seen on people that overthinks everything or just on dumb guys.
Pearson 1: *dies while laughing-crying*
Pearson 2: Hauahahahahauahaua mental diarrhea
by Kolombooo June 16, 2021
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