An eating disorder that can occur up to a year, but usually a month before Prom. Mainly occuring in young women, characterized by a self-induced starvation in order to fit into their Prom dresses or feel better about buying a slightly smaller size (which never works). Exercise in the form of "Prom night sex" usually follows Promorexia.
Will: "Dude, none of the girls are eating lunch."
John: "Yeah man, it's a month to Prom: Promorexia season."