- Used as a placeholder word or metasyntactic variable.
From the seminal text adventure game Adventure (c.1975), where it served as a "magic word"; possibly originated as a mnemonic for the subscripts of the cross product in mathematics.
xyzzy - Computer Definition
A magic word in the Colossal Cave Adventure game that lets you move instantly between distant places if entered at the right time. Because the game would respond with "Nothing Happens" if entered at the wrong time, the word was used as a mnemonic for a no-op (no operation) instruction in Data General's AOS operating system. It is also a popular variable name in programming. See no-op and mnemonic.