If you reply 'Over my dead body' when a plan or action has been suggested, you are emphasizing that you dislike it, and will do everything you can to prevent it.
[informal, emphasis]
'Let's invite her to dinner.'—'Over my dead body!'
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over my dead body
said to mean that you dislike a plan or action that has been suggested, and will do everythingyou can to prevent it
They will get Penbrook Farm only over my dead body.