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.