"divested of a body, free from flesh," of a soul or spirit, "separated from a body," 1742, past-participle adjective from disembody.
1630s, "a ghost, specter, disembodied spirit" (earlier as larve, c. 1600), from Latin larva (plural larvae), earlier larua "ghost, evil spirit, demon," also "mask," a word from Roman mythology, of unknown origin; de Vaan finds a possible derivation from Lar "tutelary god" (see Lares) "quite attractive semantically."
Crowded out in its original sense by the zoological use (1768) which began with Linnaeus, who applied the word to immature forms of animals that do not resemble, and thus "mask," the adult forms.
On the double sense of the Latin word, Carlo Ginzburg, among other observers of mythology and folklore, has commented on "the well-nigh universal association between masks and the spirits of the dead."
Old English gast "breath; good or bad spirit, angel, demon; person, man, human being," in Biblical use "soul, spirit, life," from Proto-West Germanic *gaistaz (source also of Old Saxon gest, Old Frisian jest, Middle Dutch gheest, Dutch geest, German Geist "spirit, ghost"). This is conjectured to be from a PIE root *gheis-, used in forming words involving the notions of excitement, amazement, or fear (source also of Sanskrit hedah "wrath;" Avestan zaesha- "horrible, frightful;" Gothic usgaisjan, Old English gæstan "to frighten").
Ghost is the English representative of the usual West Germanic word for "supernatural being." In Christian writing in Old English it is used to render Latin spiritus (see spirit (n.)), a sense preserved in Holy Ghost. Sense of "disembodied spirit of a dead person," especially imagined as wandering among the living or haunting them, is attested from late 14c. and returns the word toward its likely prehistoric sense.
Most Indo-European words for "soul, spirit" also double with reference to supernatural spirits. Many have a base sense of "appearance" (such as Greek phantasma; French spectre; Polish widmo, from Old Church Slavonic videti "to see;" Old English scin, Old High German giskin, originally "appearance, apparition," related to Old English scinan, Old High German skinan "to shine"). Other concepts are in French revenant, literally "returning" (from the other world), Old Norse aptr-ganga, literally "back-comer." Breton bugelnoz is literally "night-child." Latin manes probably is a euphemism.
The gh- spelling appeared early 15c. in Caxton, influenced by Flemish and Middle Dutch gheest, but was rare in English before mid-16c. Sense of "slight suggestion, mere shadow or semblance" (in ghost image, ghost of a chance, etc.) is first recorded 1610s; sense of "one who secretly does work for another" is from 1884. Ghost town is from 1908. Ghost story is by 1811. Ghost-word "apparent word or false form in a manuscript due to a blunder" is from 1886 (Skeat). Ghost in the machine was British philosopher Gilbert Ryle's term (1949) for "the mind viewed as separate from the body." The American Indian ghost dance is from 1890. To give up the ghost "die" was in Old English.
parting salutation, 1860, of unknown origin, perhaps from a German idiom (compare German parting salutation adieu so lange, the full sense of which probably is something like "farewell, whilst (we're apart)"); or perhaps from Hebrew shalom (via Yiddish sholom). Some have noted a similarity to Scandinavian leave-taking phrases, such as Norwegian Adjø så lenge, Farvel så lenge, Mor'n så lenge, literally "bye so long, farewell so long, morning so long;" and Swedish Hej så länge "good-bye for now," with så länge "for now" attested since 1850 according to Swedish sources. Most etymology sources seem to lean toward the German origin. So long (adv.) "for such a long time" is from late Old English.
Earlier guesses that it was a sailors' corruption of a South Pacific form of Arabic salaam are not now regarded as convincing. "Dictionary of American Slang" also adds to the list of candidates Irish slán "safe," said to be used as a salutation in parting. The phrase seems to have turned up simultaneously in America, Britain, and perhaps Canada, originally among lower classes. First attested use is in title and text of the last poem in Whitman's "Leaves of Grass" in the 1860 edition.
An unknown sphere, more real than I dream'd, more direct, darts awakening rays about me — So long!
Remember my words — I may again return,
I love you — I depart from materials;
I am as one disembodied, triumphant, dead.
Whitman's friend and fan William Sloane Kennedy wrote in 1923:
The salutation of parting — 'So long!' — was, I believe, until recent years, unintelligible to the majority of persons in America, especially in the interior, and to members of the middle and professional classes. I had never heard of it until I read it in Leaves of Grass, but since then have quite often heard it used by the laboring class and other classes in New England cities. Walt wrote to me, defining 'so long' thus: "A salutation of departure, greatly used among sailors, sports, & prostitutes — the sense of it is 'Till we meet again,' — conveying an inference that somehow they will doubtless so meet, sooner or later." ... It is evidently about equivalent to our 'See you later.' The phrase is reported as used by farm laborers near Banff, Scotland. In Canada it is frequently heard; 'and its use is not entirely confined to the vulgar.' It is in common use among the working classes of Liverpool and among sailors at Newcastle-upon-Tyne, and in Dorsetshire. ... The London Globe suggests that the expression is derived from the Norwegian 'Saa laenge,' a common form of 'farewell,' au revoir. If so, the phrase was picked up from the Norwegians in America, where 'So long' first was heard. The expression is now (1923) often used by the literary and artistic classes.