A simple device for preventing the accidental useage of a device. (e.g. plastic covers over fire alarm switches so that they can't be set off accidentally.)
The Jargon File defines a molly-guard as:
" A shield to prevent tripping of some Big Red Switch by clumsy or ignorant hands. Originally used of the plexiglass covers improvised for the BRS on an IBM 4341 after a programmer's toddler daughter (named Molly) frobbed it twice in one day. Later generalized to covers over stop/reset switches on disk drives and networking equipment. In hardware catalogues, you'll see the much less interesting description “guarded button”."
Yeah, the janitor put molly-guards over all the fire alarm buttons in the sports hall, so now we can't set them off by slamming them with a ball.
Can still set them off if you lift the cover though...