There are actually two common idioms based around the phrase a kettle of fish. One is yours, which means “This is a different matter from the one previously mentioned”. The other is more of an exclamation: either as a pretty kettle of fish! or a fine kettle of fish!, meaning that some awkward state of affairs has arisen. The latter is much older, dating from the eighteenth century, while yours is twentieth-century and seems to be derived from it.

Nobody is really sure where the expression comes from, but we do know that the phrase a kettle of fish was originally a literal term. These days, especially in Britain and Commonwealth countries, we think of a kettle as a small enclosed container with a handle and spout for boiling water to make our tea. (I believe that Americans are less familiar with this essential item of kitchenware.) In the eighteenth century, though, a kettle was any large vessel used to boil stuff in.

There was, it seems, a custom by which the gentry on the Scottish border with England would hold a picnic (though that term was not then known) by a river. The custom was described by Thomas Newte in his Tour of England and Scotland in 1785: “It is customary for the gentlemen who live near the Tweed to entertain their neighbours and friends with a Fete Champetre, which they call giving ‘a kettle of fish’. Tents or marquees are pitched near the flowery banks of the river ... a fire is kindled, and live salmon thrown into boiling kettles”.

What puzzles scholars is how this literal reference became an idiom — assuming, of course, that the phrase comes from the custom, which is far from certain. There is a clue in early examples, in which the term was used in the sense of a mess, muddle or confusion caused by one’s own misguided actions. For example, in Captain Francis Grose’s Dictionary of the Vulgar Tongue of 1811, it’s explained like this: “When a person has perplexed his affairs in general, or any particular business, he is said to have made a fine kettle of fish of it”. And a little later, Thomas Chandler Haliburton of Nova Scotia used it the same way in his Clockmaker: “There’s an end to the Clock trade now, and a pretty kettle of fish I’ve made of it, haven’t I? I shall never hear the last on it”.

Could it be that the contents of the kettles of fish looked messy after the fish had broken up under the influence of the boiling water? It would make sense of the early examples. But that’s just a guess.

Subscriber Henk Rietveld wrote to say that he had heard, while working in Newfoundland, that kettle of fish was a corruption of quintal of fish, a measure either of 100 pounds or a hundredweight. This is possible, since quintal was also known in the forms kintal and kentle in Newfoundland and New England, the last of which could easily have been misheard as kettle. It can’t be ruled out as a possibility, since the quintal was the usual way of measuring fish catches. Against it is the important point that the idiom kettle of fish seems to have been known first in Britain but that kentle is an American form.