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It means so crazy it makes your brain leak out your ears. You use it when something is so shocking it should be illegal.
"You just got suspended for drawing a mustache on the principal's face? That's galbonzo!" #schoolfail #facehair
"He ate the entire pizza box in one bite. Galbonzo." #foodporn #horrormovie
"I failed my math test and my dog died. That's galbonzo." #lifeisawful #mathhate