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 BSD 4.2 distribution. The joke was removed in later releases once commercial sites started using 4.2, but apparently restored on the 4.4BSD tape and in {Net,Free,Open}BSD. 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 humorless 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.”

[It has since been pointed out that indeed you can tunafish. Usually at a canning factory... —ESR]