A more thorough explanation:

Son assault demesne is a French term that means "his own assault." It refers to the plea of self-defense in a tort action. In this plea, the defendant claims that the plaintiff initiated an assault, and the defendant used only the necessary force to repel the plaintiff's assault and protect their person and property.

For example, if someone attacks another person, and the attacked person fights back to protect themselves, they can use the plea of son assault demesne in court. They can argue that they only used the necessary force to defend themselves and their property.

The law of self-defense allows a person to use force to protect themselves, their family, or their property from a real or threatened attack. However, the force used must be necessary and reasonable. If a person makes an honest but unreasonable mistake that force is necessary to repel an attack, it is called imperfect self-defense. On the other hand, if a person accurately appraises the necessity and the amount of force to repel an attack, it is called perfect self-defense.

International law also recognizes the right of a state to defend itself against a real or threatened attack. This is known as the right of self-defense.