Green’s Dictionary of Slang

John n.

also john
[commonness of the name; note police/legal jargon John Doe, any anonymous male suspect, victim etc.]

1. [late 16C+] a generic term for a man.

2. [18C–1900s] (also Johnnie, Johnny) a male servant.

3. [early 19C–1900s] (US) an Englishman [abbr. John Bull n.1 ].

4. [mid-19C–1940s] (also John Chow, Johnny) a derog. term for a Chinese man [abbr. John Chinaman n.].

5. [mid-19C+] a general term of address, orig. of white men by immigrants etc, irrespective of actual name, e.g. Hello, John, got a new motor etc.

6. [late 19C+] (S.Afr.) a generic term for any male black servant.

7. [1900s–50s] (US) a jack in poker [punning on the name Jack].

In compounds

john farmer (n.) [sense 1 above + SE farmer]

[1900s–70s] (US) an ordinary farmer.

john-hold-my-staff (n.) [note sense 2 above]

[17C] a servile attendant; also attrib.