Green’s Dictionary of Slang

1. (US black) a hard-working black man, tough and indomitable in the face of appalling challenges.

1928

N.I. White Amer. Negro Folk-Songs 190: John Hardy and John Henry were both steel-driving men, probably Negroes, in West Virginia. [Ibid.] 191: [reported from Auburn, Ala., 1915–1916] John Henry had a little woman, / And her name was Polly Ann.

2. the penis [var. on John Thomas n. (1)].

c.1888		Stag Party 217: ‘John Henry’ is hard as a bone .
1944		in PADS.
a.1968		‘School Ma’am on the Flat’ in G. Logsdon Whorehouse Bells Were Ringing (1995) 54: He took his dallies from around horn and opened her hondoo / He took John Henry in his hand and placed it in her fat.
1974		P. Gent North Dallas Forty 152: She sucked me off ’til John Henry was achin’.
1994		G. Indiana Rent Boy 73: He starts rubbing his ass lips against the head of my john henry with a corkscrew motion.
2002		S. Phillips Walkaway (2003) 268: Poor old John Henry throbbing along to my heartbeat.

3. (US) a signature or name, esp. on some form of legal or otherwise official document [var. on John Hancock n.].

1914		DN IV 109: John Henry or John Hancock. Autograph [DA].
1933		Buckner ‘Ranch Diction of the Texas Panhandle’ in AS VIII:1 31: john henry. Signature. A cowboy never signed a document; he put his John Henry to it.
1946		Mezzrow & Wolfe Really the Blues 141: He’s a cinch to put his John Henry on the dotted line.
1962		P. Crump Burn, Killer, Burn! 181: You’re supposed to mark your John Henry in it [i.e. a dance-card].
1969		B. Jackson Thief’s Primer 132: You don’t have to put up any cash [...] all you do is write your John Henry.
1977		(con. 1960s) D. Goines Black Gangster (1991) 36: We’ll just put his or her John Henry in our little black book.
2011		F. Bill ‘Cold, Hard Love’ in Crimes in Southern Indiana [ebook] ‘Shit’s up with the John Henry about his fighter’s body?’.