Thought you might like the latest question on Ask Figaro:
Please tell me where “different kettle of fish” comes from.
Sigh. Mucking out the origins of idioms is not Figaro’s bag, baby (to coin an idiom). But this one is interesting.
A “kettle of fish” once referred to a fancy riverside picnic (the original river was the Tweed, in Britain) where jolly al frescans tossed live salmon into boiling pots of water.
“Different kettle of fish” is a variation of “fine kettle of fish” or “rare kettle of fish.” They’re an ironic way of saying, “This is no picnic.”
Put that in your pot and boil it.