When you say that something will only happen ‘over my dead body,’ you mean that you are willing to do all you can to prevent it.
Example of Use: "All of my friends are going out to the lake tonight and I'm going too!" Answer: "Over my dead body you are!"
While not much is known about the origin of the idiom 'over my dead body,' scholars know that it was in use during the 1800s, when it was a common hyperbolic expression that was often used in jest. As you may know if you’ve used this idiom, it is one that can be used jokingly or with consternation, depending upon the circumstances.