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

barney

   In Commonwealth hackish, "barney" is to fred as bar is to
   foo.  That is, people who commonly use "fred" as their first
   metasyntactic variable will often use "barney" second.  The
   reference is, of course, to Fred Flintstone and Barney Rubble
   in the Flintstones cartoons.

   [Jargon File]

   (1994-11-28)


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

barney
 n.

    In Commonwealth hackish, barney is to fred (sense #1) as bar is to foo
    . That is, people who commonly use fred as their first metasyntactic
    variable will often use barney second. The reference is, of course, to Fred
    Flintstone and Barney Rubble in the Flintstones cartoons.