Green’s Dictionary of Slang

1. a respectable member of society.

1933		Ersine Und. and Prison Sl.
1935		D. Lamson We Who Are About to Die 195: The square johns don’t know enough to talk nice to the bulls an’ get ’em to listen to reason.
1947		Waterloo (IA) Daily Courier 19 Jan. 35/1: Double-talking in America was spread by cheap vaudevillians trying to gum up Square Johns in small towns.
1959		(con. c.1915) G. Duffy Warden’s Wife 72: Fugitives [...] would [...] forcibly persuade some ‘square John’ into an exchange of clothing.
1965		B. Jackson Get Your Ass in the Water (1974) 85: An old squarejohn seen the play come off and he run and told the men.
1967		(con. 1920s) J. Thompson South of Heaven (1994) 223: A square John going crooked just when a smart sheriff turns stupid!
1973		E. Bunker No Beast So Fierce 92: Woven into my compassion was disgust at Stan’s incompetence. A squarejohn watching Dragnet would avoid Stan’s blunders.
1986		R. Campbell In La-La Land We Trust (1999) 56: I could read inside the heads of all those square johns.
1990		(con. early 1950s) J. Ellroy L.A. Confidential 242: Eyewitness accounts: squarejohns at the beach that night. Nice folks.
2001		(con. 1964–8) J. Ellroy Cold Six Thousand 195: We use civilians, exclusive. Squarejohns who owe casino markers.
2013		J. Stahl Bad Sex on Speed 135: The three of us went buzzing out to watch the square-johns come to test their eyes on us.
2021		J. Ellroy Widespread Panic 52: I saw three big squarejohns in gray suits.

2. (US Und.) in criminal terms, a dependable, reliable person.

1921		Nebraska State Jrnl (Lincoln, NE) 16 Oct. 31/6: He held out his hand, wrung Square John Kelly’s in a hard grip [...] ‘Kelly’s all right [...] Never turns down ol’ friend in hard luck. Square John — betcher life!’.
1949		N. Algren Man with the Golden Arm 184: Good guys and their true-blue pals, the real sports and square johns.
1962		C. Clausen I Love You Honey, But the Season’s Over 172: Bob became known as ‘Good Smith’ and his brother as ‘Bad Smith.’ Good Smith was a ‘square John’ (hard worker) [...] His brother, Bad, was a self-confessed shirker and crook.

3. (drugs) one who eschews drugs.

1935		A.J. Pollock Und. Speaks n.p.: Square John, a dope peddler who is not addicted.
1937		B. Dai Opium Addiction in Chicago 173: Their attitudes toward what they call the ‘square Johns’ or non-users.
1949		Monteleone Criminal Sl. (rev. edn).
1959		J.E. Schmidt Narcotics Lingo and Lore.

4. (US gay) a heterosexual man who enjoys the company of lesbians.

1972

B. Rodgers Queens’ Vernacular 71: square john (this one, however, doesn’t want to make it with a dyke, he just wants to bar-hop with them.).