As we are in the midst of NCAA football season, I can't think of a better time to bring up the compound word sidewalk alumni. Though sidewalk alumni is not listed in traditional dictionaries yet, it is semantically similar to, and brings to mind the compound word armchair quarterback, which is listed in dictionaries. Sidewalk alumni and armchair quarterback are semantically similar in that they are both considered derogatory terms for sports fans.
Sidewalk alumni: Fans of college sports teams that are not alumni of the college.
Armchair quarterback: Viewers of sports who criticize plays and think they could play better.