A senior or upperclassman that is an outcast within their own grade, so they resort to being friends with underclassmen i.e Freshman.
Freshmongers are usually female, most often obese and hated by their own classmates. Freshman look up to them because they are upperclassmen and think they are cool for hanging out with one.
It is very likely that the Freshmonger won't have a date to the prom, so they invite one of their Freshman buddies so they won't be so lonely.
Freshman Girl 1: OMG rnt we sooo cool? We hng out with Seniors! Maybe they will invite us to prom!
Freshman Girl 2: We r soo hot! Let's go be sluts!
Freshmonger: Do you guys want to go to prom with me? Nobody wants to be my date!! Isn't that weird? I'm so hot!