MOS is a standard filmmaking jargon acronym used in production reports to indicate an associated film segment has no synchronous audio track.
The production sound mixer on stage connected the control panel to the recording room and the camera.
According to this theory, a German director, recently transplanted to Hollywood (probably Ernst Lubitsch, but possibly Fritz Lang), was asked by a script supervisor how he would like to shoot the next scene of the day.
which translates to "without speaking", and so this was noted as a joke on the production reports and the camera slates for the shot.
In The Screenwriter's Bible, author David Trottier credits the term to Austrian director Erich von Stroheim, who allegedly would tell his crew "Ve'll shoot dis mid out sound.
"[4] Other explanations for the meaning of the initials have appeared over the years in books, articles, publications, journals, and web pages:[1] The terms "MOS," "motor only shot," "mit out sound," or any of their versions, do not occur in the following works: Additionally, the term(s) do not occur in any book published in the United Kingdom[citation needed] on motion picture production or post-production methods and procedures.