Green’s Dictionary of Slang

1. (US, also honest Joe) an honest citizen, a hard-working person.

1600		Munday & Drayton Sir John Oldcastle I ii: In the mean space this remains For kind sir John of Wrotham, honest Jack.
1884		R.C. Hartranft Journal of Solomon Sidesplitter 148: Never did Paddy utter a better bull than did an Honest John, who, being asked by a friend, ‘Has your sister got a son or daughter?’ answered, ‘Positively I do not yet know whether I am an uncle or an aunt.’.
1933		J. Spenser Limey 31: The gangsters of to-day are just mercenaries harrying and exploiting the [...] ‘honest Johns’ of modern America.
1947		E. Condon We Called It Music 182: If I had a job I would be in working clothes—tuxedo—and the honest johns on their way to day labor gave me hard looks.
1959		J. Thompson Getaway in Four Novels (1983) 28: He’d dealt with Honest Johns before, and they’d never turned out as pure as they were supposed to be.
1966		C. Wood Fill the Stage With Happy Hours (1967) Act VII: They’re not all like me you know, honest John.
1991		B. Hamper Rivethead (1992) 80: There was much more to getting yourself than being an honest Joe.
2013		K. Sampson Killing Pool 51: I can never love this tribe they call Scouse; even the honest Johns whose good name Hodge is fighting for.

2. (US prison) a naïve person who does not know how to ‘work the system’.

1935

D. Lamson We Who Are About to Die 191: The honest-johns get soaked plenty years because they’re honest an’ green an’ don’t know the business.

3. (US und.) as sense 1, but working as a legitmate front for criminals.

1955

B. Appel Tough Guy [ebook] John Terry, a politician on the Spotter’s payroll (the Spotter suspected his Honest John of being in on two or three other tinboxes).