Source : Free On-Line Dictionary of Computing

Moof
     
        /moof/ [MAC users] 1.  A semi-legendary creature, also called
        the "dogcow", that lurks in the depths of the {Macintosh}
        Technical Notes {Hypercard} stack V3.1; specifically, the full
        story of the dogcow is told in technical note #31 (the
        particular Moof illustrated is properly named "Clarus").
        Option-shift-click will cause it to emit a characteristic
        "Moof!" or "!fooM" sound.  *Getting* to tech note 31 is the
        hard part; to discover how to do that, one must needs examine
        the stack script with a hackerly eye.  Clue: {rot13} is
        involved.  A dogcow also appears if you choose "Page Setup..."
        with a LaserWriter selected and click on the "Options" button.
     
        2. Used to flag software that's a hack, something untested and
        on the edge.  On one {Apple Computer} {CD-ROM}, certain {folder}s such
        as "Tools & Apps (Moof!)" and "Development Platforms (Moof!)",
        are so marked to indicate that they contain software not fully
        tested or sanctioned by the powers that be.  When you open
        these folders you cross the boundary into hackerland.
     
        [{Jargon File}]