Word/phrase meaning fatal blow (in an argument)

I'm searching for a word, phrase or term that means the fatal blow, something along the lines of coup de grace. However, that term has implies a merciful killing. I'm looking for something that connotes a savagery; a complete evisceration.

For example, you're describing the arguments an attorney is making to a court. The attorney is hammering the adversary. Then s/he makes this [coup de grace] argument to just drive it home. There's nothing merciful about it (that's why I don't like coup de grace in this context).

Thank you.

Solution 1:

You're right that the traditional meaning of coup de grace is merciful killing, but I also hear it when it's intended to mean a general final blow or end. Many may consider this usage incorrect, and you can avoid it if you want, but here are a few sources that give this more general meaning:

2:a decisive finishing blow, act, or event
The decision to cut funding is the coup de grâce to the governor's proposal.
Merriam-Webster Dictionary

2.A finishing stroke or decisive event.
American Heritage Dictionary

2.a final or decisive stroke
Collins Dictionary

2.any finishing or decisive stroke.
Random House Kernerman Webster's College Dictionary

Same definition as above from Random House Unabridged (dictionary.com)

A word or term that's related to coup de grace is "death blow", sometimes given as one word, which has less association with putting out one's misery or mercy killing.

1.1An event or circumstance which abruptly ends something.
Oxford Living Dictionaries

You can see a few examples of its use in the last link to see if this is satisfactory to you.

Solution 2:

"deathblow" (or "death blow") seems like a good fit.

From Collins - "If you say that an event or action deals a death blow to something such as a plan or hope, or is a death blow to something, you mean that it puts an end to it."

• The deportations would be a death blow to the peace process.
• It has been asserted that the theory of evolution dealt a death blow to religious belief.
• The death blow came after the American government decided not to intervene to save the bank.

From MW"death blow" or "deathblow" - "an act or event that causes the end or failure of something."