The term "over my dead body" is commonly used to show that someone is vehemently opposed to something. It's commonly used in conversation, either in a joking manner or as an ultimatum. The difference between usage is normally shown by body language and/or the method of vocalization.

Sometimes in literature it can be used as something ironic or for comedic effect.

  Sheriff: You're coming with me to the jailhouse.
  Desperado Jim: Over my dead body!
  Sheriff fast-draws his six-shooter and fills Desperado Jim full of holes.
  Sheriff: As you wish, Princess.

If used as an ultimatum, make sure you know the other person well enough such that they don't take you literally.

Iron Noder 2017