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
Some college students are using the word loyalty as a synonym for monogamy. Are the meanings of these words now shifting? Plus, a biologist discovers a new species of bat, then names it after a poet he admires. Also, warm memories of how a childhood...
Why would some Spanish speakers use adaptations of certain English words when perfectly good Spanish words for the same thing already exist? Plus, handy terms in a dictionary of the Sussex dialect from 150 years ago: Back then, a dezzick was “a...
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