The phrase John Hancock to refer to a signature is derived from the name of the American revolutionary John Hancock (1737 - 1793) who was the first to sign the American Declaration Of Independence.
Just tack this on to Jabba's answer. It just goes a little more in depth.
When the declaration was signed, all the men signed VERY tiny so that the King might not be able to tell who they were, but John Hancock stepped up and signed his name bold and large. That's why people always say, "Sign your John Hancock!"