John
early 20th century (in john (sense 2)): from the given name John, used from late Middle English as a form of address to a man, or to denote various occupations, including that of priest (late Middle English) and policeman (mid 17th century).
wiktionary
From the male given name John ( q.v.), whose ubiquity led to extensive use of the name in generic contexts. Its use for toilets derive from John and Cousin John, which both probably relate to jacques and jakes, used in equivalent senses by the British and Irish. [1]
etymonline
John
masc. proper name, Middle English Jon, Jan (mid-12c.), from Old French Jan, Jean, Jehan (Modern French Jean), from Medieval Latin Johannes, an alteration of Late Latin Joannes, from Greek Ioannes, from Hebrew Yohanan (longer form y'hohanan), said to mean literally "Jehovah has favored" or "Jah is gracious," from hanan "he was gracious."
Greek conformed the Hebrew ending to its own customs. The -h- in English was inserted in imitation of the Medieval Latin form. Old English had the Biblical name as Iohannes. As the name of John the Baptist and John the Evangelist, it was one of the most frequent Christian given names, and in England by early 14c. it rivaled William in popularity and was used generically (in Middle English especially of priests) and as an appellative (as in John Barleycorn, John Bull, John Q. Public). Somehow it also became the characteristic name of a Chinaman (1818).
The Latin name also is the source of French Jean, Spanish Juan, Italian Giovanni, Portuguese João, also Dutch Jan, Hans, German Johann, Russian Ivan. Welsh form was Ieuan, Efan (see Evan), but Ioan was adopted for the Welsh Authorized Version of the Bible, hence frequency of Jones as a Welsh surname.
john (n.)
"toilet," 1932, probably from jakes, used for "toilet" since 15c. Meaning "prostitute's customer" is from 1911, probably from the common, and thus anonymous, name by which they identified themselves. Meaning "policeman" is by 1901, from shortening of johndarm (1823), a jocular Englishing of gendarme.
"John Darm! who's he?" "What, don't you know!
In Paris he is all the go;
Like money here,—he's every thing;
A demigod—at least a king!
You cannot fight, you cannot drink,
Nor have a spree, nor hardly think,
For fear you should create a charm,
To conjure up the fiend John Darm!
["John Darm," in "Varieties in Verse," John Ogden, London, 1823]
