According to Oxford, “stan” can be both a verb and a noun and it denotes “an overzealous or obsessive fan of a particular celebrity.” The dictionary even cites Eminem’s song as the origin of the term.
For those that don’t remember, “Stan” is written in the form of fan letters to Eminem from his biggest fan. At first, the letters are very pleasant and fawning, but when his idol doesn’t write back right away, Stan becomes violently angry and believes the fantasy scenarios Eminem raps about are real.
In the years since, as fandom culture has grown on the internet, “stan” has become an easy term for overzealous fans who are always eager to defend their pop idols. Now it’s official.