In a long, detailed scene in Bernard Cornwell's Agincourt, Sir John's armorer helps his lord don his armor for the impending battle. The scene ends thus:

. . .Sir John flexed his arms and found everything satisfactory, and so gave a grim smile. "Thank you, Cartwright," he said.
. . .Cartwright bowed his head and spoke the words he had always spoken, right from the very first time he had armored his master. "Sir John," he said, "you are dressed to kill."
. . .As were thirty thousand Frenchmen.

Could this sort of usage be the origin of the phrase dressed fit to kill?

Partridge suggests it came from the US in 1856. That's over four hundred years after the setting of the book.

I sure couldn't find much about it.