Green’s Dictionary of Slang

citizen n.

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

1. [mid–late 19C] (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 ).

2. [20C+] (US) a person, the implication is of a respectable individual as opposed to a criminal.

3. [1910s+] a rough, poss. criminal person.

4. [1970s] (US gay) in specific use of sense 2, a heterosexual.