Green’s Dictionary of Slang

(US) one’s signature, esp. on some form of legal or otherwise official document.

[1833		C.A. Davis Letters of Major J. Downing (1835) 20: It [i.e. a signature] is as much like the original handwritin of Major Downing, as old John Hancock’s is of hisn to the Declaration of Independence].
1889		C.C. Post Ten Years A Cowboy 338: I say, stranger, [...] would you mind tellin’ me if this is your ‘John Hancock’?
1906		‘Hugh McHugh’ Skidoo! 41: They would have waited for a snow storm on the 16th of January before signing their John Hancocks.
1913		Day Book (Chicago) 24 June 31/1: We’ll feel better [...] when Gov. Dunne sticks his John Hancock on that suffrage bill.
1914		DN IV 109: John Henry or John Hancock. Autograph.
1922		S. Lewis Babbitt (1974) 69: Put your John Hancock on that line.
1931		Irwin Amer. Tramp and Und. Sl. 111: John Hancock. – A signature, from the patriot of that name who set his name to the Declaration of Independence.
1939		Pittsburgh Courier (PA) 25 Nov. 7/1: I will draw your John Hancock on the carrier, [...] lay it on Farley’s stooge, who in turn will mitt it [i.e. a letter] to you.
1947		(con. 1944) J.H. Burns Gallery (1948) 337: I’m so sick of signing my John Hancock on the lower left-hand corner of packages that are being mailed home.
1962		Ragen & Finston World’s Toughest Prison 805: john hancock – A signature.
1971		in DARE File n.p.: If you’ll just put your John H. here [...] [=on a work order].
1976		R. Sabbag Snowblind (1978) 236: A good penman, [...] can forge a signature as blatantly not his as John Hancock’s without raising suspicion.
1999		H. Hock Forever Green 162: Well Eddie, you’d have to put your John Hancock on a piece of paper stating you’d never deface anyone’s property again.
2005		in T. Kordyban More Hot Air 60: Put your John Hancock down on the bottom line so we can broom this out.
2014		M. Sheridan ‘192 Over 110’ in ThugLit Mar. [ebook] I told Hing if [...] he just put his greasy old John Hancock down on the order form, I would personally give up my cut.