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An umbrellasite is someone who, unaware of the day's weather conditions, comes to school without an umbrella and begs others to come under their umbrella in between classes, somewhat irritatingly.
Guy 1: "Shit I forgot to bring an umbrella... Hey dude can I come under there?"
Guy 2: "No. Fuck off, you umbrellasite."
by Riyakou April 19, 2009