Source : Free On-Line Dictionary of Computing

tunafish
     
         In hackish lore, refers to the
        mutated punchline of an age-old joke to be found at the bottom
        of the manual pages of "tunefs(8)" in the original {4.2BSD}
        distribution.  The joke was removed in later releases once
        commercial sites started using 4.2.  Tunefs relates to the
        "tuning" of {file-system} parameters for optimum performance,
        and at the bottom of a few pages of wizardly inscriptions was
        a "BUGS" section consisting of the line "You can tune a file
        system, but you can't tunafish".  Variants of this can be seen
        in other BSD versions, though it has been excised from some
        versions by humourless management droids.  The [nt]roff source
        for SunOS 4.1.1 contains a comment apparently designed to
        prevent this: "Take this out and a Unix Demon will dog your
        steps from now until the "time_t's wrap around."
     
        [{Jargon File}]
     
        (1997-01-12)