A shonenfag is a person who loves shonen (anime or manga for kids, with a very simple plot, nearly always about who's the most powerful and they last during decades (for example,
DragonBall,
one of the most popular shonen started in 1984 and they're still making new episodes today. ) a little too much and they're saying
anything else is bad even without knowing anything about it. Similar to narutard . They're the opposite of weeb and they're sometimes referenced as normies. They often insult people when they find good arguments who demean their favorite shows.