"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].
as in the real McCoy, "the real thing; the genuine article," by 1881, said to be from Scottish the real Mackay (1883), which is of uncertain origin, though there are many candidates, the most likely of which is that it refers to whiskey distilled by A. and M. Mackay of Glasgow (the phrase the real McCoy became popular during Prohibition to describe liquor). Other stories credit it to Charles S. "Kid" McCoy (1872-1940), former welterweight boxing champ; and to a claimant for chief of the northern branch of the clan Mackay.
"By jingo! yes; so it will be. It's the 'real McCoy,' as Jim Hicks says. Nobody but a devil can find us there." [James S. Bond, "The Rise and Fall of the Union Club," Yorkville, Canada, 1881]
mid-14c., "a plotting of evil, unlawful design; a combination of persons for an evil purpose," from Anglo-French conspiracie, Old French conspiracie "conspiracy, plot," from Latin conspirationem (nominative conspiratio) "agreement, union, unanimity," noun of action from past-participle stem of conspirare "to agree, unite, plot," literally "to breathe together" (see conspire).
Earlier in same sense was conspiration (early 14c.), from French conspiration (13c.), from Latin conspirationem. An Old English word for it was facengecwis.
Conspiracy theory "explanation of an event or situation involving unwarranted belief that it is caused by a conspiracy among powerful forces" emerged in mid-20c. (by 1937) and figures in the writings of, or about, Charles Beard, Hofstadter, Veblen, etc., but the degree of paranoia and unreasonableness implied in each use is not always easy to discern. The phrase was used from 19c. in a non-pejorative sense "the theory that a (certain) conspiracy exists," especially in court cases. Its use in general reference to theories of hidden cabals pulling wires behind the scenes of national or global events is by 1871.
We shall better understand the ensuing civil war if we study the movements in the four most important of these States, in relation to a theory which asserts that the secession was a conspiracy whose central cabal, composed of Southern senators and representatives in Washington, dictated through its ramifications in the States the inception and the course of the revolution. [James Ford Rhodes, page headed "The Conspiracy Theory" in "History of the United States from the Compromise of 1850," New York, 1893]
To the Jingo Imperialist "the South African Conspiracy" is the alleged Dutch conspiracy to drive the British into the sea. But, to the man accustomed to weigh evidence and to base his opinions on ascertained facts, it is clear that this conspiracy theory is absolutely untenable, for whatever "evidence" has been adduced in support of the theory is nebulous and shadowy in the extreme. ["The South African Conspiracy," in The Westminster Review, January 1902]