To act and or be stupid.
Gregg "Dude I'm crazy cool!"
Zach "No your a dumb ass Kool-Aid Drinker!"
One who accepts an argument or philosophy wholeheartedly and blindly.
The term is in reference to the religious cult led by Jim Jones, who committed mass suicide by drinking Kool-Aid laced with Valium and cyanide.
You'll never convince Mike not to buy that MacBook even though he just got a G4, he's such a kool-aid drinker.