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When an organization or community is doing poorly enough that its smartest people, seeing the writing on the wall, leave for better opportunities elsewhere. This is common in business and other organizations that perpetuate a common goal or mission and once purged it typically accelerates the demise of the organiztion.

For example: Facebook's poor stock performance since it's IPO and lack of favorable stock options for employees puts them at risk of brain drain. Many will likely head over to Google or Apple, but they'll be in trouble.
Mark Zuckerberg: Sheryl, can find me those repots on Zynga's...Hey?! Why are wearing a Google sweatshirt?!

Sheryl Sandberg: Sorry Mark. We had a good run, but your lack of focus on ad revenue and overvalued IPO has caused this drop in FB stock value with no end in sight. I and the others are leaving before the stock tanks.

Mark Z: The others? They're all leaving Facebook? You can't! We'll never survive this brain drain!

Sheryl: Yeah, sorry. You should've thought about that before going public.
by Tenacious Faulker August 03, 2012
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When you study way to hard and your brain is drained.
"Dude, I had 3 exams to study for over the long weekend and I pushed off all my studying till 10:30 last night"

"Woahh, you must have major brain drain!!"

"heck yes"
by Josh1800 October 10, 2006
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when after jacking off for a long time or having sex for a long time and you finally cum and the load that you blow is so immense that you can feel your sack emptying out leaving the skin of it so tight that it constricts around your balls leaving your sack looking like a brain.
Dude i sobered up halfway through and saw how not hot she was and it ended up taking me so long to finish i definitely had some brain drain.
by J Dude June 18, 2010
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When one car window is open and you can feel pulsing in your ears. Use the back window to maximize the effect.
to experience the brain drain we roll down the window and feel the flapping of our ear drums in the wind.
by sallicehte August 04, 2006
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To Snipe Someone In The Head In A Video Game Or I Guess In Real Life.

It Has To Be A Sick Shot Before You Can Drop The Term Any Normal Snipe Is Not Good Enough.
*Tom Headshots A Rookie In A Game Of Halo*
Tom: Ohhh Shit What A Phat Brain Drain.
Frined: That Was a Fucking Sick Shot
by Jonathan R Potts December 05, 2007
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The process of getting an exceptional blowjob, hence the word “brain”, and letting loose the largest load of all time.
Jake: “Yo, Andrew did you get it from Angie last night.”

Andrew: “Yeah man, she gave me the brain drain.”
by Trident of Treason August 24, 2018
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Going on and on like a bloodstain phrase written by Marianne Faithful. Rambling so much you empty the contents of another person
She was such a brain drain babbling on as she was
by Dgflyinca August 27, 2018
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