A fan boy* (or fan boi) is a slang term for someone who is considered very devoted to a single subject, often to the point where it might be considered an obsession. It is a term reserved for when the obsession or passion is beyond what just being a typical fan might be considered as being.
Traditionally they were a passionate fan of something in geek culture such as a sci-fi genre, comics or specific video games, but more recently it has been expanded to include non-geek niches. It is also considered to be a type of insult or put down and is somewhat derogatory.
Fan boys* typically go into an outburst or start insulting people with ad hominem attacks when what they like or are passionate about is questioned; they often resort to many of the logical fallacies to defend their topic, especially in the context of being confronted with scientific evidence of a problem with the topic. They are also well known for a lack of objectivity in relation to the topic or niche. They tune out to facts and other opinions. Usually arguments or debates with them are futile and non-productive because of this lack of objectivity and critical thinking skills.
*or fan girl.
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