A portmanteau of Gym (abbreviation of Gymnasium) and Imogen.
Jakob: Probably at the gym.
Sam: Classic Gymogen.
Imogen: Hey guys, sorry I'm late. I was in the gym.
Flatmates in unison: Gymogen!
Sam: Where's Gymogen?
Imogen: (walking in) I've been in the pool all day.
Sam: Maybe we should call you Swimogen!
(Awkward silence ensues)