Willow,
When colonists in American published the American Declaration of Independence in 1776 it was considered an act of treason by the British government.
Anyone who signed the Declaration was, of course, considered a traitor and marked for death by the British.
John Hancock was the first to sign this historic document. He not only signed it, his signature was made with such large letters that it was immediately visible to everyone. (He also had very beautiful handwriting.)
Since that time in America "put your John Hancock here" and other similar expressions have come to mean ....sign your name......write your signature. It honors the courage of John Hancock.