Tbh I thought the origin of this term was 4chan just like all words ended in -fag like youngfag, furfag, newfag, etc. But I don't remember reading it in an English speaking context, always Spanish. The meaning is obvious and it's usually used for people who constantly bitch that movies and videogames of their childhood were so much better than the modern stuff but they're not being objective and are disrespectful of different opinions. For example if I said "Ninja Turtles cartoon from 1980s is so much better than all modern cartoons" I would be a nostalfag because when you grow up you realize that show is so bad even if you loved it when you were a child.
