xyzzy /X-Y-Z-Z-Y/, /X-Y-ziz'ee/, /ziz'ee/, or /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<enter><right-shift>' 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 diasappeared 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.
Another possible pre-ADVENT origin is discussed at the XYZZY page.