I checked the Oxford English Dictionary (OED) about the claim that Mark Twain invented the word "Injun." Not true. The first recorded use of "Injun" was by someone named J. Cocke in an August 1812 issue of the Salem Gazette.
But Twain's Tom Sawyer (1876) is the first recorded use of the phrase "Honest Injun." Someone named Talcott had written the words "Honest Indians" a couple of centuries earlier, but only in their literal sense. Twain may have been the first to use "Honest Injun" as an exclamation meaning, "No, really. I'm not kidding!"
Of course, being the first citation in the OED doesn't prove Twain invented the phrase. Twain may have seen or heard it somewhere the OED couldn't document. But Twain certainly gets credit for making the phrase popular.
For more on "Injun," see Calling BS on Steele's "Injun" and "Injun" as Bad as N-Word? For more on Mark Twain, see Mark Twain, Indian Hater.