When you absolutely will not allow a person to do something you might say "over my dead body." The [literal] meaning is that you will stand in the way of that person and only if they kill you and walk over your dead body will they be able to do what they want. Example: "You kids are not going out in the cold without your jackets! Not over my dead body!" Parents say "over my dead body" often because they are in a position of authority and they are much bigger than their children. Example: "All of my friends are going out to the lake tonight and I'm going too!" Answer: "Over my dead body you are!"