MST

From PPC Wiki

MST stands for Mystery Science Theatre, originally a popular television programme. In the show a guy called Joel (and later, a guy called Mike) was trapped in a satellite with a bunch of talking (and sarcastic) robots - namely, Tom Servo, a small but thick red torch with a gumball head and a soothing baritone, and Crow, a crazy, semi-cosplaying robot with funny eyes and a bowling pin beak. His bosses back on Earth (both Dr. and Pearl Forrester) 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, generally as an attempt to save themselves from complete madness (though, said madness has claimed Crow at least once).

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