Usually means an obsessed superfan but have evolved to mean someone with unwavering support of a person or entity. Came from the Eminem song "Stan" about a crazy fan.

https://en.wikipedia.org/wiki/Stan_(song)

>The name of the eponymous character has given rise to a slang term online which refers to overzealous, maniacal, overly obsessed fans of a celebrity or personality; the term has been included in the Oxford English Dictionary.

something like "be a fan boy of..."

https://www.urbandictionary.com/define.php?term=Stan