Museum-Mit Out Sound
Museum-MOS Mit Out Sound
A popular origin theory is that MOS stands for broken English "mit ohne sound", that is, "without sound" as a 1920s German-émigré director might have said it.
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. The director apparently responded "mit ohne sprechen!" 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, David Trottier credits the term to Austrian director Erich von Stroheim, who allegedly would tell his crew "Ve'll shoot dis mid out sound." (Wikipedia)