Etymology
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watchword (n.)
also watch-word, c. 1400, "password," from watch (n.) in the military sense of "period of standing guard duty" + word (n.). In the sense of "motto, slogan" it dates from 1738.
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Dieu et mon droit 

French, "God and my right," the watchword of Richard I at the Battle of Gisors (1195), adopted as the motto on the royal arms of England. The "right" was Edward's claim to the crown of France upon the death of his uncle, Charles the Fair, king of France, without male issue.

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countersign (n.)

"a military watchword, a signal given to a soldier on guard, with orders to let no one pass who does not first give that signal," 1590s, from French contresigne, from contre- "against" (see contra-) + signe "sign" (see sign (n.)).

COUNTERSIGN. A watchword used by military bodies as a precaution against an enemy or enemies. The countersign may be changed at any moment, or any number of times, but is usually altered each twenty-four hours. It is given primarily to commanders of guards, and outposts and their sentries, to reconnoitring and visiting patrols, and to the field and regimental officer of the day. All others desiring to pass through the lines must first be supplied with the countersign, which is thus a guard against spies, strangers, and surprise. ["New International Encyclopaedia," 1906]
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symbol (n.)

early 15c., "creed, summary, religious belief," from Late Latin symbolum "creed, token, mark," from Greek symbolon "token, watchword, sign by which one infers; ticket, a permit, licence" (the word was applied c. 250 by Cyprian of Carthage to the Apostles' Creed, on the notion of the "mark" that distinguishes Christians from pagans), literally "that which is thrown or cast together," from assimilated form of syn- "together" (see syn-) + bole "a throwing, a casting, the stroke of a missile, bolt, beam," from bol-, nominative stem of ballein "to throw" (from PIE root *gwele- "to throw, reach").

The sense evolution in Greek is from "throwing things together" to "contrasting" to "comparing" to "token used in comparisons to determine if something is genuine." Hence, "outward sign" of something. The meaning "something which stands for something else" first recorded 1590 (in "Faerie Queene"). As a written character, 1610s.

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jingo (n.)

"mindless, militaristic patriot," 1878, picked up from the refrain of a music hall song written by G.W. Hunt, and sung by "Gilbert H. MacDermott" (1845-1901), supporting aggressive British policy toward Russia at a time of international tension. ("We don't want to fight, But by Jingo! if we do, We've got the ships, we've got the men, We've got the money too.")

Hunt's patriotic song of 1878, with a swinging tune ... became at Macdermott's instigation the watchword of the popular supporters of England's bellicose policy. The "Daily News" on 11 March 1878 first dubbed the latter 'Jingoes' in derision .... ["Dictionary of National Biography," London, 1912]

As an asseveration, by jingo was in colloquial use from 1690s (high jingo is attested from 1660s), and jingo here is apparently yet another euphemism for Jesus (it translates French Par Dieu), influenced by conjurer's gibberish presto-jingo (attested from 1660s). The frequent suggestion that it somehow derives from Basque Jinkoa "god" is "not impossible" but is "as yet unsupported by evidence" [OED].

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shibboleth (n.)

late 14c., the Hebrew word shibboleth, meaning "flood, stream," also "ear of corn;" in Judges xii.4-6. It was the password used by the Gileadites to distinguish their own men from fleeing Ephraimites, because Ephraimites could not pronounce the -sh- sound. Hence the figurative sense of "watchword" (first recorded 1630s), which evolved by 1862 to "outmoded slogan still adhered to." A similar test-word was cicera "chick pease," used by the Italians to identify the French (who could not pronounce it correctly) during the massacre called the Sicilian Vespers (1282).

During training exercises on Pavuvu and Guadalcanal, the need to improve battlefield security is to be implemented not by a simple password, but by an identification procedure described as "sign and countersign." The ground rules are to sequentially interrogate an unknown friend or foe with the name of an automobile, preferably one with an "L" in its vocalization. The response is to be a cognomen for another automobile uttered in the same manner. This insures the "friend" entering our lines will reply with the correct countersign in a dialect distincly American; call out "Cadiwac" or "Chryswer," and you're dead. [Perry Pollins, "Tales of a Feather Merchant: The World War II Memoir of a Marine Radioman, 2006]
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