1
general::
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]
American Heritage Idioms
2
general::
Phrase(s): one’s John Hancock
Go to one's John Henry and one's John Hancock
McGrawhill's American Idioms And Phrasal Verbs