[fragmede]

Tuna is a type of fish so the joke is that they sound the same.

[SoftTalker]

There was also the record You Can Tune a Piano, but You Can't Tuna Fish by REO Speedwagon, I had assumed that's what the tunefs man page was referencing as that was a best-selling record at around the time Unix was being developed.

[throwanem]

Yes. It had been a popular joke in the US, I believe about a generation before. (Probably the joke came in around the same time as canned tuna.)