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
Jim in Columbia, South Carolina, has noticed sportscasters’ use of the phrase on the season with reference to a period of time. This construction shows up in the sports pages as early as the 1880s, first referring to a team’s revenue and...
In the 15th century, the word respair meant “to have hope again.” Although this word fell out of use, it’s among dozens collected in a new book of soothing vocabulary for troubled times. Plus, baseball slang: If a batter...
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