sidewalk alumni n.pl. fans of a sports team from a school or college they never attended or have no direct ties to. (source: Double-Tongued Dictionary)
Further reading
In 1971, when a new public library opened in Troy, Michigan, famous authors and artists were invited to write letters to the city’s youngest readers, extolling the many benefits of libraries. One of the loveliest was from E.B. White, author of...
“What has a head like a cat, feet like a cat, a tail like a cat, but isn’t a cat?” Answer: a kitten! A 1948 children’s joke book has lots of these to share with kids. Plus: an easy explanation for the difference between...
There’s another variation, which I have heard most commonly applied to Notre Dame fans—subway alumni.
My guess is that fans in the stands are composed of more sidewalk/subway alumni than any other.
Alabama @ Tuscaloosa has the larget sidewalk alimni fanbase in the nation…as well as the most mullets per capita