Top Definition
A temporary reduction in mental capacity due to having recently run several miles.
After my run yesterday, I spent half an hour wandering around the grocery store trying to decide what to eat for dinner, only to remember that I had just gone to the grocery store that morning. So then I went to In-n-Out. I think I'm coming down with a case of Runner's Brain.
by An Avid Runner April 08, 2013

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