It comes from the phrase "deserved desserts" which refers to the sweet treat one would earn for good behavior. Over time, "deserved" was dropped, and the phrase became "just desserts," with the "just" referring to fairness and appropriateness.
Here are some examples of how it's used:
* "After cheating on his exams, he got his just desserts when he failed the class."
* "She worked tirelessly for years, and finally got her just desserts when she was promoted."
* "The company's unethical practices eventually led to their just desserts when they were forced to close."
It's important to note that "just desserts" can refer to both positive and negative consequences, depending on the context.