As the story goes, when acclaimed expressionist/noir master Fritz Lang of Metropolis fame, fled Nazi Germany and began making films in the United States, his professional success in his new country outpaced his mastery of the English language. His focus on purely visual film making, to the exclusion of the relatively new development of sync sound, combined with his habit of articulating his thoughts in a hodgepodge pidgin tongue of English and German let to the accidental adoption of the acronym M.O.S..
Supposedly, Lang was so in the habit of shooting "Mit out sound," mit being the German equivalent of the English preposition "with," that this expression became part of the industry vernacular. There are a number of such terms, "meat-bag," "Gary Coleman," "C-47," "Abby Singer" and "10-100" all spring readily to mind and they all have legends to accompany them. In this case, though, the popular legend is a myth.
When scenes are shot on celluloid film, as opposed to on video, the visuals and the audio are captured by two entirely independent devices and are then combined in post production. Because these are two separate elements, either can be run without the other. Sounds can be recorded without rolling the camera and picture can be shot without any accompanying audio. When we do the latter, it's called shooting "MOS." There are a number of reasons to do this, it's a static shot with no dialog, an insert of something that makes no sound, the creative decision to edit in music with no diagetic audio has already been made, contaminating sounds will make the sound track useless etc.
In the early days of sync sound film making, the late 20's and early 30's, sound was applied to film using an optical ribbon. On the bits filmed without sound, the script supervisor would label the shot, Minus Optical Stripe, informing the editor that there was no need to go searching for the audio component.
I have also heard that, before we had electrical motors as sophisticated as we do today, the camera and the audio machinery had to run off of the same 220volt circuit. If for some reason, the camera had to be run on a different electrical channel, matching up the two elements would become very complicated, if not outright impossible. Ergo, such shots were photographed without audio and labeled "Motor Off Sync."
It's my suspicion that both stories are, to some degree, true. Among location audio professionals the first is the generally accepted tale.
In either case, I do hope that this Mit Out Sound nonsense is put to bed.
You have thusly been enlightened.