John Hancock

[After John HANCOCK (from the prominence of his signature on the Declaration of Independence).]

Also, John Henry. One's signature, as in Just put your John Hancock on the dotted line. This expression alludes to John Hancock's prominent signature on the Declaration of Independence. The variant simply substitutes a common name for "Hancock." [Mid-1800s]

The noun has 2 meanings:

Meaning #1: American revolutionary patriot who was president of the Continental Congress; was the first signer of the Declaration of Independence (1737-1793)
  Synonym: Hancock

Meaning #2: a person's own signature
  Synonym: autograph