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2.
When it takes one's brain a few seconds to catch up with their actions.
Person A: Dude, you just tried to drink a sandwich.
Person B: Huh? Oh great... Brain Lag.
by Kick to da ballz July 15, 2011
 
1.
The sleep-deprived, often disorienting way a person feels after working on so many impending deadlines their life has become a blur, usually made possible by the regular consumption of several litres of caffeinated beverage in short amounts of time.
Huh? Oh, I get it now. Sorry: brain-lag!

Employer: "Have you been here all night? You look like shit!"
Employee: "Yeah, a bad case of brain-lag..."
{employer leaves cubicle}
Employee: "...bastard."
by Adam Schilling February 16, 2006