Short for Mystery Science Theater 3000 - to dissect a fic line by line and ridicule it. Some people view MSTing as comedy, others view it as flaming.
Inspired by the cable series MYSTERY SCIENCE THEATER 3000, MSTing a story means taking someone else's story and adding in your own smart-alec comments, mocking it. One might consider it Ms. Nitpicker out of control. Unlike Ms. Nitpicker, an MST doesn't generally keep the victim anonymous. It can be very funny, but also very cruel. The standard version uses the actual narrators from the real MST 3000, mocking the story as they mock bad movies on their series, but other writers use characters from the story's universe--for instance, Harry Potter might insist he would never behave that way. Personal attacks on the author of the original badfic are sometimes part of the piece.
An acronym that references the popular cult tv show, Mystery Science Theater 3000. Variations include: mst3k, mst. "Dudes, we have to mst Titanic." Pegasus Jones The Smash's fabulously gay alter ego."Dude, you are so pegasus jones. Quit whining."