Footage recorded without sound but which is intended for use in a sound film; the appropriate noises are added later in production. If the footage involves an actor speaking and thus has to be dubbed, expect issues with synchronizing the voice track.
The source of the initialism is debated; theories include "Motor Only Shot/Sync" (per TOW's article), "Missing Optical Sound", "Motion Omit Sound" and "Mit Out Sprechen."
See also Looping Lines.
(Note: Some poster or another might've camelcased the abbreviation for Man of Steel into this. Was that what you were looking for?)