A more thorough explanation:

A casual affray, also known as chance-medley or chaud-medley, is a spontaneous fight during which one participant kills another in self-defense. The term chance-medley comes from the Anglo-Norman language and means "chance scuffle".

For example, if two people get into a fight and one person pulls out a knife, the other person may use deadly force to defend themselves. If the person with the knife is killed in the process, it may be considered a casual affray.

It's important to note that the killing must be in self-defense and not premeditated. The term is often misused to describe any accidental killing, but it specifically refers to a killing that occurs during a sudden brawl or quarrel.