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Noun - The occurrence when you have stayed up late enough at night that you not only forget what day it is, but also think that what you did mere hours ago actually happened the day before.
Tom: "Hey, Katie said you messaged her three times tonight, I think you should take the hint, dude."
Bill: "Oh, crap. I forgot I already had! Bad case of dayfusion, man."
Tom: "Maybe you shouldn't stay up so late?"
by Roberto' August 07, 2012
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