A made-up word that people use when they don't know the singularity of "alumnus" or plurality of "alumni." Often used openly by educational institutions.
Guy 1: Dude, that dude's an alum.
Guy 2: A what?
Guy 1: An alum, dude.
Guy 2: What's that?
Guy 1: You know, bro, he graduated from here.
Guy 2: Don't you mean he's an "alumnus"?
Guy 1: No, dude, wtf? Alum. Just like cact and fung.