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A molly-guard is a physical shield or barrier that prevents unauthorized or improper access to part of a hardware device. The molly-guard is placed over a button, lever or other control so that less experienced users have more trouble getting to it and it cannot be activated by accident.
In the urban legends surrounding this term, “molly-guard” is often attributed to a programmer working on an IBM 4341 mainframe computer. According to the story, the programmer's young daughter, who was named Molly, pushed a particularly important control button twice during a given day. Presumably, the programmer then constructed some type of molly-guard to prevent further difficulty.
As the term has emerged, it has often been used in conjunction with the term “big red switch” for a primary power switch, shutdown switch or other primary control, and may be used on hardware devices or even on industrial machinery.