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
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Quiz Guy John Chaneski insists that the names of modern Olympic sports resulted from changing one letter in the original names for each activity. For example, he says, the pole vault must have originated from an earlier sport in which participants...
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