colloquial for "signature," 1903 (sometimes, through some unexplainable error, John Henry), from the Boston merchant and rebel (1736-1793), signer of the U.S. Declaration of Independence. The extended sense is from his signing that dangerous document first or most flamboyantly.
The family name is attested from 1276 in Yorkshire, a diminutive (see cock) of Hann, a very common given name in 13c. Yorkshire as a pet form of Henry or John.