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Someone who is so closely involved with certain fandoms (whether it be a book series, a tv show, a celebrity, etc etc) that it completely destroys their mental health and social life. Someone who can't go 10 minutes without thinking about their obsession and crying silently to themselves. These types of people have had their lives taken over, so don't act surprised when you find their secret shrine at the back of their closet.
"Hey, is Emma coming to the party tonight?"

"Nope, she's gonna be in her room for the rest of the night watching Twilight reruns."
"Oh yeah, I forgot she was an Over-Obsessive Fangirl."
via giphy
by Mh188 August 11, 2018
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