phrase request - Is there an English equivalent for the Italian saying "It's another pair of sleeves"?

In a meeting an Italian told a puzzled English audience: "It's another pair of sleeves".

It's an Italian way of saying: "it's another thing", or "this new argument is something different or off topic". The phrase could be used to remark that the new argument is something different or off topic. I don't think it can be used for pointing something unique.

In the context of the meeting it was used to note that the new argument was about a completely different thing with no connection with the previous one. The expression comes from the medieval use of interchangeable sleeves in woman dress.

There is some similar colorful way of saying this meaning ?

Answer

In British English, you can indicate that two subjects, things or situations are completely different by saying about one of them:

That's another kettle of fish
That's a different kettle of fish

It can be used verbatim, on its own, in various circumstances. If, for example, somebody brings up a subject that, in your opinion, is nothing to do with the current topic of conversation.

It can also be used to emphasise that the second subject is considerably more demanding than the first, for example:

Lend you a hundred pounds? No problem: but ten thousand pounds? That's a different kettle of fish altogether!

You can also use it to make it clear that you consider two things or situations are completely different. Here are two examples:

Practical (or everyday) intelligence seems to be a different kettle of fish from academic intelligence Mechanisms of everyday cognition

When it's your own [son], that's a different kettle of fish. A beautiful death