(noun) When you are sad or nostalgic about a show that you have performed in has ended, or you miss the cast and crew members that you are now finished working with. Happens often when the cast of actors and actresses live away from each other, in different towns or even states.
Person 1: Hey are you feeling alright?
Person 2: I have a pretty bad case of showsickness.
Person 1: Why?
Person 2: I miss everyone from the show I was just in.