Green’s Dictionary of Slang

john n.2

1. a carman, a coachman.

2. in Und. uses, a victim, a source of money.

(a) [1900s–60s] (US Und., also james) an easy victim, a sucker; a free spender.

(b) [1910s+] (US Und.) any law-abiding man.

(c) [1910s+] (orig. US) a female or male prostitute’s client.

(d) in attrib. use of sense 1(c).

(e) [1940s–60s] (US black) a gullible white man.

(f) [1980s] (US black) a man susceptible to feminine trickery.

3. [1910s] (Aus.) a boyfriend.

4. [1910s+] the penis [abbr. John Thomas n. (1)].

5. [1920s+] a skilled, professional tramp [such a top-class tramp is well dressed and thus resembles a ‘normal’ citizen].

6. [1930s+] (orig. US college) the lavatory, usu. for men [? abbr. cuz john n.].

7. [1950s+] (gay) an older man who supports a younger one without actually sharing a long-term relationship with him.

8. [1960s+] (US) one’s signature [abbr. John Hancock n.].

9. [1960s+] (US gay) among lesbians, a man who associates with female homosexuals.

10. [1960s+] a condom. [note johnny n.1 (12)].

11. [1970s] (US) the menstrual period.

12. [1990s+] (US drugs) heroin [fig. use of John n. (1), based on boy n.2 (4a) and the idea that heroin is a ‘masculine’ drug].

In compounds

john catcher (n.)

[1950s] (US) a prostitute.

john walker (n.) [John n. (1) + SE walk, i.e. to remove]

[1970s] (US Und.) a security man in a brothel.

In phrases

john among the maids (n.)

[19C] a whoremonger, a promiscuous man.

play john (v.)

[mid-19C] of a man, to flirt.