Someone who immerses themselves in other people's fandoms to gain recognition, respect, or favours from them.
Being a fandom slut is a sorry state of affairs, it means you have no mind of your own. It also serves to really, really piss people off, because they think they've met someone who likes what they like, whereas as a matter of fact, they're just having their fandoms screwed over.
Person A: "I love Dynasty Warriors! The fighting is so cool!"
Person B: "Really? *spends three hours looking up disturbed Dynasty Warriors fanart just to impress Person A*"
Person A: *some time later* "Hey, watcha lookin' at?"
Person B: "This!" *waves some mentally-scarring Cao Pi X Sima Yi drawings in Person A's face*
Person A: *feels dead inside"

Person B is a fandom slut.
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