Green’s Dictionary of Slang

(US) respectable, upright; esp. when pertaining to a non-criminal world or sensibility; also adv (see cite 2023).

1950		Goldin et al. DAUL 205/2: Square-John pinch. 1. An arrest for a crime not specifically of an underworld character, as rape, desertion, and non-professional fraud.
1966		F. Elli Riot (1967) 36: A squarejohn wife-killer like Skinny.
1973		E. Bunker No Beast So Fierce 169: He was a squarejohn citizen, a believer in the death penalty, a coward, a dog.
1994		Rebennack & Rummel Under A Hoodoo Moon 119: For all the square-goods dudes I met like Joe, there were ten who’d gone wrong.
2007		(con. 1962) in E. Bunker Stark 10: He’s out to screw the squarejohn world.
2023		(con. 1962) J. Ellroy Enchanters 68: My squarejohn attaché is a toolkit. I’m dressed squarejohn.