late 14c., "cloudy, misty, hazy" (of the eye, fire-smoke, etc.), from Latin nebulosus "cloudy, misty, foggy, full of vapor," from nebula "mist, vapor" (from PIE root *nebh- "cloud"). The figurative sense of "hazy, vague, formless" is attested by 1831. Astronomical sense, in reference to stars or star clusters surrounded by luminous haze, is from 1670s. Related: Nebulously; nebulousness.
It forms all or part of: nebula; nebular; nebulosity; nebulous; Neptune; Nibelungenlied; Niflheim; nimbus.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit nabhas- "vapor, cloud, mists, fog, sky;" Greek nephele, nephos "cloud;" Latin nebula "mist, vapor, fog, smoke, exhalation;" German Nebel "fog;" Old English nifol "dark, gloomy;" Welsh niwl "cloud, fog;" Slavic nebo.
"nebulous, hair-like envelope surrounding the head of a comet," 1765, from Latin coma, from Greek komē "hair of the head," which is of unknown origin. Earlier in English as a botanical term for a tuft of hairs (1660s). For the constellation Coma Berenices, see Berenice. Related: Comal.
1855, theater slang for "a failure in performance;" by 1862 it had acquired the general sense of "any ignominious failure or dismal flop," on or off the stage. It comes via the French phrase faire fiasco "turn out a failure" (19c.), from Italian far fiasco "suffer a complete breakdown in performance," literally "make a bottle," from fiasco "bottle," from Late Latin flasco "bottle" (see flask).
The literal sense of the image (if it is one) is obscure today, but "the usual range of fanciful theories has been advanced" [Ayto]. Century Dictionary says "perhaps in allusion to the bursting of a bottle," Weekley pronounces it impenetrable and compares French ramasser un pelle "to come a cropper (in bicycling), literally to pick up a shovel." OED keeps its distance and lets nameless "Italian etymologists" make nebulous reference to "alleged incidents in Italian theatrical history." Klein suggests Venetian glass-crafters tossing aside imperfect pieces to be made later into common flasks. But according to an Italian dictionary, fare il fiasco used to mean "to play a game so that the one that loses will pay the fiasco," in other words, he will buy the next bottle (of wine). If the dates are not objectionable, that plausibly connects the literal sense of the word with the notion of "a costly mistake."
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]