Definition
Used to emphatically state that a decision is final and not open to further discussion or change, regardless of any attempts to persuade otherwise.
就这么说定了，没有商量的余地。
I've decided to buy this car, and that's flat!
我决定买这辆车，就是这样！
We are not working overtime this weekend, and that's flat!
我们这个周末不会加班，就是这样！
I won't lend you any more money, and that's flat!
我不会再借钱给你，就是这样！