Definition
This idiom is used to firmly state that a decision is final and will not be changed, despite any attempts to persuade otherwise.
就这么说定了，没得商量！
I've made up my mind to buy this car, and that's flat!
我已经决定要买这辆车，这事就这么定了！
We are going camping next week, and that's flat!
我们下周要去露营，这事就这么定了！
I will not lend you any more money, and that's flat!
我不会再借你钱了，这事就这么定了！