Believed to be a Western United States cowboy variation (dating from the late 1700s or early 1800s) of John Hancock, the first person to sign the United States Declaration of Independence and whose name later became a synonym for a signature. This usage predates the American folklore hero John Henry from the 1870s.
- (idiomatic) One's signature.
1933, American Dialect Society, American Speech, volume 8, page 31:
- JOHN HENRY. Signature. A cowboy never signed a document; he put his John Henry to it.