Green’s Dictionary of Slang

square-john adj.

also square-goods
[square john n.]

(US) respectable, upright; esp. when pertaining to a non-criminal world or sensibility.

[US]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.
[US]F. Elli Riot (1967) 36: A squarejohn wife-killer like Skinny.
[US]E. Bunker No Beast So Fierce 169: He was a squarejohn citizen, a believer in the death penalty, a coward, a dog.
[US]Rebennack & Rummel Under A Hoodoo Moon 119: For all the square-goods dudes I met like Joe, there were ten who’d gone wrong.
[US](con. 1962) in E. Bunker Stark 10: He’s out to screw the squarejohn world.