Why do we say "Over My Dead Body"?

Meaning:

I will not allow that to happen

Background:

This expression appears to date back to at least the 18th century but, according to Google's Ngram viewer (which tracks references in books by time period), gained popularity around the 1850s, and has substantially gained in popularity since the start of this century.

It is almost always used in the first person, even in reported speech. For example: "The official rhetoric from the business community toward any tax increase continues to be an over-my-dead-body 'no'" —Boston Globe, 1989

It is unclear on how Google factors in the much larger number of books available today compared to previous centuries when calculating the frequency with which an expression appears. There appears to be some weighting given because, if there was none, every expression would see a substantial uptick in usage in recent decades, but that is not the case.

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