A new kettle of fish dating
As Peter Shor's comment beneath Ralph Richardson's answer indicates, "kettle of fish" has been used as a slang term for several centuries.The same definition of the term that he points to appears in Francis Grose, . I doubt [that is, fear] we have but rouz'd a sleeping Lion : A stop-Thief has sometimes saved a House-breaker ; and many a Wench has sav'd her Reputation by crying Whore first : But the more this Matter is stirred, the more it stinks, and I doubt we have made a fine Kettle of Fish on't.'A different kettle of fish' is much later in origin than 'a pretty kettle of fish' and is known only since the 1920s.
It means 'something different from the thing before'." It is a question for the commentator on proverbial expressions, whether this application of " Since "another kettle of fish" and "a different kettle of fish" emerged as idioms more than a century after "a fine/pretty kettle of fish" did, the meaning of the new phrases was probably at least tinged with the notion that the metaphorical kettle of fish in question might be problematic.That is, "another kettle of fish" didn't simply mean "another thing altogether" but "another thing—and one that may be troublesome to put in order." This sense of the phrase is supported by its first Google Books appearance, in 1860. "I received the despatch only two hours ago." "Here is another kettle of fish," continued the lady, musing.The latter expressions seem very likely to have emerge from the former ones, the shift in meaning appears to have be swift and substantial.With such like discourses they consumed near half-an-hour, whilst Betty provided a shirt from the hostler, who was one of her sweethearts, and put it on poor Joseph.
Something like disease there—and a nice variety of it, too! A short time ago I heard one of them preaching in a field before thousands and thousands of people.