John Henry is an American folk hero and tall tale. Henry worked as a "steel-driver"—a man tasked with hammering and chiseling rock in the construction of tunnels for railroad tracks. In the legend, John Henry's prowess as a steel-driver was measured in a race against a steam powered hammer, which he won only to die in victory with his hammer in his hand. The story of John Henry has been the subject of numerous songs, stories, plays, and novels. source: http://en.wikipedia.org/wiki/John_Henry_(folklore)

Look him up on Google.

A John Henry is used as a slang term for a signature. However, it is wrong. The correct slang term a signature on a document is a John Hancock. On the American Declaration of Independence, a significant historical document declaring independence of the American colonies from England, the first and largest signature was that of John Hancock. After that, a John Hancock came to mean someone's official signature. "Just put your John Hancock on the document and the house is yours." So when people talk about a John Henry (an American folk hero), they really mean a John Hancock. For more information, go here: http://www.fold3.com/page/41_the_signature_of_john_hancock/