Who coined the phrase “true grit”?

Bing Copilot says it came earlier than 1897 – sometime in the early 19th century.

“The phrase “true grit” originated in early 19th-century American English. The word “grit” in this context refers to strength of character, stamina, and determination. The notion likely comes from the idea of “gritting one’s teeth” as a sign of resolve and perseverance.”

Everything else I saw related to the book and movie.

I doubt you would ever find the answer to this one. I suspect it is as @smudges mentioned that it came from a usage of the word “grit”. Yes, someone used “true grit” first, but trying to find that will likely be impossible.

^^True. The great google wouldn’t be able to help. With some strange luck maybe, somebody wrote down in a by now very old book the one who used the phrase first and I would stumble on it.