The Free On-line Dictionary of Computing (19 January 2023):

molly-guard

   /mol'ee-gard/ [University of Illinois] 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 generalised
   to covers over stop/reset switches on disk drives and
   networking equipment.

   [Jargon File]


The Jargon File (version 4.4.7, 29 Dec 2003):

molly-guard
 /mol'ee·gard/, n.

    [University of Illinois] 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”.