When one is overcome with a desire to be exceptionally muscular. Often results in long disappearances to the gym, excessive protein intake, large grunting noises, lots of standing in front of the mirror.
A:Yo bro-bot, I am going to the gym.
B:Again! Yesterday you went for like 5 hours. Maybe you're megarexic.
A: Megarexia would be good for my biceps. ::flexes::
A:Where did you go?
B: I ate a steak... I gotta go look at how huge my muscles are getting, excuse me.
A: Dude, you have megarexia!