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
“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...
The edge of the Grand Canyon. A remote mountaintop. A medieval cathedral. Some places are so mystical you feel like you’re close to another dimension of space and time. There’s a term for such locales: thin places. And: did you ever go...
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