Green’s Dictionary of Slang

citizen n.

[SE; note citizen, a civilian as opposed to a soldier, used by Shakespeare in Coriolanus (1607)]

1. (UK Und.) a wedge used for opening safes; thus citizen’s friend, a smaller form of wedge (cf. alderman n. (5); gentleman n.; lord mayor n.1 ).

[UK]Farmer & Henley Sl. and Its Analogues.
[UK]R.T. Hopkins Banker Tells All 137: ‘We have a citizen – that’s a small one – and a citizen’s friend, which is smaller still.’ Casey’s safe-breaking technique was the last word in the London underworld.

2. (US) a person, the implication is of a respectable individual as opposed to a criminal.

[US]E. Hemingway letter 4 Sept. in Baker Sel. Letters (1981) 303: Well it’s fine to hear you citizens are married.
[US]W.R. Burnett Iron Man 33: Twenty men crowded through the door [...] reporters, amateurs, pugs, hangers-on, and plain citizens.
[US]J. Archibald ‘Bird Cagey’ in Popular Detective Jan. [Internet] Well, well [...] citizens like Clarence should not get spiffed.
[US]D. Runyon Runyon à la Carte 94: I know citizens who will sit up all night making up propositions to offer other citizens the next day.
[US]W. Burroughs Naked Lunch (1968) 147: I’m fucking this citizen.
[US]E. Bunker No Beast So Fierce 169: He was a squarejohn citizen, a believer in the death penalty, a coward, a dog.
[US](con. 1967) Bunch & Cole Reckoning for Kings (1989) 64: A citizen is somebody who ain’t a brother.
[US]E. Little Another Day in Paradise 138: Anything that puts a citizen at risk is not acceptable.
[Aus]J.J. DeCeglie Drawing Dead [ebook] I thought some citizen was giving me the eye and went over and stared at him.

3. a rough, poss. criminal person.

[UK]Wodehouse Psmith Journalist (1993) 234: On these were seated as tough-looking a collection of citizens as one might wish to see.
Beckley Post-Herald (WV) 1 Dec. 7/4: Citizen — A fellow gang member.
[Ire](con. 1940s) B. Behan Borstal Boy 268: He must be keeping it [...] to scare people, which it did, but not this citizen.

4. (US gay) in specific use of sense 2, a heterosexual.

[US]B. Rodgers Queens’ Vernacular.