- that's 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!
Dictionary of American idioms. 2013.