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1.
prison slang. A "square". A working stiff. A law abiding citizen. Female: square john broad.
"He's a squarejohn. Works 9 to 5 at the factory."

Also see sucker. "A squarejohn is always an easy mark."
by Dtat January 09, 2009
 
2.
A Square John is slang meaning a reliable trust worthy guy. The term originated from Bike Gangs and is common in the Canadian Prison system. It is considered to be a compliment if you are referred to as a Square John
He's cool man, don't worry he's a Square John.
by John-999 January 31, 2011
 
3.
n. a lame-o, or. someone who displays no signs of hipness or sense of adventure whatsoever; a person who is overtly conservative, often likely to be a narc; a gaynal
Why did you ask Jason to party with us tonight? He is such a squarejohn; I heard he ratted out Phil for having a dimebag in school.
by Lord Gaynal September 26, 2006