What does "That's it!" mean? -- That's it. I'm leaving.

[Phoo]

What does "That's it!" mean in this case below:

"That's it. I'm leaving." (This person is angry.)

According to Longman on line dictionary:

"that"
used to refer to a person, thing, idea etc that has already been mentioned or is already known about:

"it"
used to refer to a thing, animal, situation, idea etc that has already been mentioned or is already known about:

Sounds like both are almost same to me.
If so, can I say "It's that!" instead of "That's it!" as the same meaning?
If I cannot, what is the difference b/w "that" and "it"?

[Yankee]

Hi Phoo

"That's it" has a fixed idiomatic meaning and is a way of saying that something is finished or has come to an end. It isn't always said in anger, but it can be.

As with most idiomatic expressions, if you alter the wording, you frequently lose the idiomatic meaning. "It's that" does not mean the same thing as "That's it".
.

[Phoo]

Hi Amy, thanks for your comment.

I have got it.

Now, how about "That's that!" ? Is this the same as "That's it!"?

[Yankee]

Hi Phoo

"That's that" is a bit similar to "That's it".

"That's that" is often used to indicate that a person does not wish to discuss something further and that as far as they are concerned, the conversation is finished. For example:

Child: Mom, can I go to the movies tonight?

Mom: No, not on a school night.

Child: But the movie ends at 9:30.

Mom: That's too late for a school night, and besides, you haven't even begun your homework yet.

Child: I can start my homework now and finish it after the movie.

Mom: You can start your homework now, but you are not going to a movie tonight.

Child: But everybody else is allowed to go.

Mom: If you want to go to that movie, you can go on Friday or Saturday.

Child: But I want to go tonight!

Mom: No! You're not going to a movie tonight and that's that!
.

[Conchita]

[Lost_Soul]

[Yankee]

Hi Alex

Yes, that's right.
.