A more thorough explanation:

Definition: To put an end to the cause. This phrase is used in legal contexts, particularly in reference to the legal effect of a peremptory defense. A peremptory defense is a defendant's stated reason why the plaintiff or prosecutor has no valid case.

Example: If a defendant claims that the plaintiff's case is invalid because the statute of limitations has expired, this is a peremptory defense. The defendant is essentially saying that the plaintiff waited too long to bring the case and therefore has no legal right to sue. By using the phrase "perimere causam," the defendant is indicating that they are seeking to put an end to the cause of action (i.e. the plaintiff's legal claim).