Meaning:

You say "That's that" when you want to say:

    1. something is ended and you cannot change it.

    2. you refuse to do something and nothing will not persuade you to do it.

Example:

They've sold their house and are moving to Europe, and that's that. (They've already decided to do so. They won't change their plan.)

I won't do that. I'm just leaving now, and that's that.

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