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."