What is necessary in a combat system?

[Cluedrew]

For example, don't call a mechanic "mordhau" when you mean "coup de grace".

Question that I don't actually know the answer to but is also makes a point. What does "coup de grace" mean? I've heard it used enough that I know what you are referring to, but the words are kind of nonsense. If I had to guess: "attack on helpless opponent" by context and "graceful overthrow" by the words.