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.
A story that makes your head explode because it could have been over in about five minutes if one or more of the character(s) didn't have an IQ below room temperature.
It can also be caused when a character who is presented as being quite smart does something uncharacteristically stupid, and everything goes downhill from there.
Despite their legendary status, Shakespeare's plays are notorious for being massive brain drainers.
One example is Romeo and Juliet, which could have been over very quickly with no deaths if the two families just communicated with each other.
Another example is Julius Caesar, which would have ended after the death of Caesar if Brutus didn't decide to save Mark Antony because "he can't think for himself," and they didn't want people thinking they were terrorists. Long story short, he eventually found out that was a bad move when Antony suddenly dropped the façade and revealed that he is much more intelligent and independent than he seems.
A turn of phrase meant to insult either yourself or others for their foolishness or stupidityto the point of it seeming as if there brain is leaking out of their own skull, hence brain drain
A friend that is ordinarily quite adept at chess, horribly blunders an obvious play, the only possible explanation being a severe brain drain at the time.
Hungario: The pawn was adjacent to your queen, there want even any threat of taking it, what happened?
Klempt: My bad, serious case of Brain Drain over here i guess.
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.