This is a sarcastic rhetorical question aimed at a remark that has no bearing on whatever is actually being discussed. Nobody expects an answer about fish, because the question is not really about fish at all. It is a way of telling someone that their last comment was a non-sequitur and that the conversation should return to the point. Fish is simply a stand-in for any dull, everyday commodity, and the phrase works just as well with eggs, tea or anything else bought by weight, since the joke depends on naming something mundane and completely disconnected from the matter at hand.
What’s the origin of the phrase ‘what does that have to do with the price of fish?’?
The phrase belongs to a family of interchangeable put-downs built on a single template: what has that got to do with the price of a given everyday item, with the blank filled by fish, eggs, tea or whatever ordinary goods come to mind. According to the etymologist Pascal Tréguer, this pattern is a colloquial, sarcastic phrase that functions as “a rhetorical question calling attention to a non-sequitur or irrelevant statement or suggestion made by another person”, and he traces the pattern to American English.
The template does not depend on any particular commodity, which is why so many versions exist side by side, with fish, eggs and tea in China among the most familiar fillers of the same joke. The logic is always identical: the price of an ordinary commodity is treated as a plain, settled fact that has nothing to do with the point under discussion, so invoking it deflates whatever irrelevant remark has just been made. Because the humour comes from the mismatch itself rather than from any real event involving fish markets or fishmongers, there is no single traceable moment when the fish version was first coined. It grew out of a broader habit of American sarcastic retorts that reach for an everyday commodity price as the measure of irrelevance, and fish, being both common and faintly comic as a subject, became one of the most lasting choices within that pattern.