Stan is an expression for overzealous fans and groupies, who are obsessed about an artist, a band or a celebrity.
Calling oneself a “Stan” can also be used to express, how greatly you like something or someone.
The expression comes from the legendary song of Eminem; “Stan”.
The song presents us with a dangerously obsessed fan of the rapper, who is writing more and more disturbing letters, as he is mentally deteriorating, since he doesn’t get a response from his idol.
He confesses to self-harm and even performs suicide, with the intent of sending it to Eminem.
The song was published in 2000, and “Stan” in its slang meaning was first used by Nas in his 2001 song Ether, where he describes Jay-Z to be a “Stan”.
The first time “Stan” was defined on Urban Dictionary as a person with an extreme fandom, was in 2006, and two years later it was already utilized in tweets, to express one’s appreciation toward a theme, or a person.
By the 2010’s the slang expression was well established in the language and people were arguing about which “Stan” community is more devoted, resulting in the creation of the website Stan Wars in 2010.
For other definitions on the subject: https://www.urbandictionary.com/define.php?term=Stan