Can either mean

Immersed in a certain topic more than usual for a long period of time.

Having trouble concentrating on your surroundings and daydreaming more than usual.

Named after Gundolf Köhler, who was known for being extremely immersed in Nazi Germany and was told to be living in his own fantasy world by his friends.
Guy 1: Hey man, why'd you get all these games out?
Guy 2: I guess it's a Gundolf day.
Guy 1: Ah, I see.

Guy 1: Hey dude, are you listening to me?
Guy 2: Uh, what? Sorry, i wasn't paying attention
Guy 1: Gundolf Day?
Guy 2: Haha, yeah.
by May 1, 2021
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