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

xyzzy
 /X?Y?Z?Z?Y/, /X?Y?ziz'ee/, /ziz?ee/, /ik?ziz?ee/, adj.

    [from the ADVENT game] The canonical ?magic word?. This comes from 
    ADVENT, in which the idea is to explore an underground cave with many
    rooms and to collect the treasures you find there. If you type xyzzy at the
    appropriate time, you can move instantly between two otherwise distant
    points. If, therefore, you encounter some bit of magic, you might remark
    on this quite succinctly by saying simply ?Xyzzy!? ?Ordinarily you can't
    look at someone else's screen if he has protected it, but if you type
    quadruple-bucky-clear the system will let you do it anyway.? ?Xyzzy!? It's
    traditional for xyzzy to be an Easter egg in games with text interfaces.

    Xyzzy has actually been implemented as an undocumented no-op command on
    several OSes; in Data General's AOS/VS, for example, it would typically
    respond ?Nothing happens?, just as ADVENT did if the magic was invoked at
    the wrong spot or before a player had performed the action that enabled the
    word. In more recent 32-bit versions, by the way, AOS/VS responds ?Twice as
    much happens?.

    Early versions of the popular ?minesweeper? game under Microsoft Windows
    had a cheat mode triggered by the command ?xyzzy? that
    turns the top-left pixel of the screen different colors depending on
    whether or not the cursor is over a bomb. This feature temporarily
    disappeared in Windows 98, but reappeared in Windows 2000.

    The following passage from The Wonderful Wizard of Oz by L. Frank Baum,
    suggesting a possible pre-ADVENT origin, has recently come to light: ?
    Ziz-zy, zuz-zy, zik!? said Dorothy, who was now standing on both feet. This
    ended the saying of the charm, and they heard a great chattering and
    flapping of wings, as the band of Winged Monkeys flew up to them.

    The text can be viewed at Project Gutenberg.

    Y


The Free On-line Dictionary of Computing (30 December 2018):

xyzzy

    The canonical "magic word" from the ADVENT
   adventure game, in which the idea is to explore an underground
   cave with many rooms and to collect the treasures you find
   there.  If you type "xyzzy" at the appropriate time, you can
   move instantly between two otherwise distant points.  If,
   therefore, you encounter some bit of magic, you might remark
   on this quite succinctly by saying simply "Xyzzy!"
   "Ordinarily you can't look at someone else's screen if he has
   protected it, but if you type quadruple-bucky-clear the system
   will let you do it anyway."  "Xyzzy!"  Xyzzy has actually been
   implemented as an undocumented no-op command on several OSes;
   in Data General's AOS/VS, for example, it would typically
   respond "Nothing happens", just as ADVENT did if the magic
   was invoked at the wrong spot or before a player had performed
   the action that enabled the word.  In more recent 32 bit
   versions, by the way, AOS/VS responds "Twice as much happens".
   See also plugh.

   [Jargon File]