Green’s Dictionary of Slang

1. (orig. US) an outsider, one who is not a part of a given group, e.g. of a gang.

1974		R. Blount Jr About Three Bricks Shy of a Load 37: [A]lthough he was built much too well to be considered fat by civilian [i.e. non-pro footballers] standards, he had a great tendency to put weight on.
1975		H. Ellison Deadly Streets (1983) 15: The juvies waged war against each other, and ‘civilians’ were pretty much exempt from the slaughter.
1984		B. Ellem Doing Time 189: civilian: a person from outside the prison system.
1989		J. Morton Lowspeak 39: Civilian – someone who is neither a police officer nor a working criminal.
2018		Unknown T ‘Mad about Bars’ 🎵 Fuck civilians.
2024		C. Stella Joey Piss Pot 33: ‘We have enough problems without whacking some civilian over this bullshit’.

2. (US gay) a heterosexual.

1972		B. Rodgers Queens’ Vernacular.
1979		Maledicta III:2 218: Though much rejected themselves, gays reject straights as commoners and civilians, enforce some conformity within their own ranks (with terms to distinguish and describe what is politically correct).