To give your John Hancock

First Example:

Ken: I can’t believe that I have been offered a position to play in the Los Angeles Dodgers Baseball club.

Dan:  All you have to do is write your John Hancock on the contract and you will be eight million dollars richer.

Ken:  I never thought my Signature would mean so much.

Dan:  You better get used to it. Now that you’re famous, you will need to use your John Hancock for everything from contracts to autographs.

Second Example:

Nancy: We can’t buy this car without giving our John Hancock.

John: Everything official needs a signature.  Giving our John Hancock is our acknowledgement of commitment.

Nancy: Yeah, that’s true.  I just wish I would have practiced a nicer looking signature.

John:  It doesn’t really matter.  You’re not famous anyway.

Meaning: The expression "John Hancock" refers to a person’s signature.  It dates back to the signing of the American Declaration of Independence.  Mr. Hancock’s signature was so large that it was very noticeable.  It’s expressed in several ways.  Some of the most common ways are: “to give your John Hancock” or “Write your John Hancock.”

http://www.languagesystems.com/