that's why

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that's why

Used after a statement to emphasize the reason or explanation just given, especially in response to a statement or question using the word "why." A: "Why can't I go to the party with my friends?" B: "Because you have a huge test on Monday that you need to study for, that's why!" A: "I don't understand why we need to learn this stuff." B: "Because it will be useful to you for the rest of your life, that's why."
See also: why

that's why

a tag on the end of a statement that is an answer to a question beginning with why. (Shows a little impatience.) Sue: Why do you always put your right shoe on first? Bob: Because, when I get ready to put on my shoes, I always pick up the right one first, that's why! Mary: Why do you eat that awful peppermint candy? Tom: Because I like it, that's why!
See also: why