MST

MST stands for Mystery Science Theatre, originally a popular television programme. In the show a guy called Joel was trapped in a satellite with a bunch of talking (and sarcastic) robots. His bosses, back on Earth sent him cheesy B Movies and monitored him while he watched them. Joel and the Robots made sarcastic, or humorous comments about the movie while watching.

In fanfiction MST refers to an author going through a piece of fanfic, and adding their own comments, as either themselves, or other characters.