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

qux

    /kwuhks/ A metasyntactic variable in the foo series;
   a mutation from quux.

   (2015-03-22)


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

qux
 /kwuhks/

    The fourth of the standard metasyntactic variable, after baz and before
    the quu(u...)x series. See foo, bar, baz, quux. This appears to be
    a recent mutation from quux, and many versions (especially older
    versions) of the standard series just run foo, bar, baz, quux, ....