ca‧su‧al‧ty, n. /ˈkæʒuəlti/
From “casual,” in the sense of a thing that is not formal. In the age of chivalry, it referred to a wound that did not require treatment. This is what Monty Python referenced with the Black Knight’s insistence that “It’s just a flesh wound!”
The usage shifted when “formality” ceased to mean a wound that required attention and started to mean something that a lawyer didn’t want you to think too hard about.