1824, originally in African-American vernacular in the South.

The slaves themselves entertain the very highest contempt for white servants, whom they designate as 'poor white trash.' [Fanny Kemble, journal, Jan. 6, 1833]

[T]he term [poor white] is rather loosely applied by Northern writers even to mountaineers and to small farmers who live on a precarious footing. But in the Southern conception, not everyone who is both poor and white is a "poor white." To the Southerner, the "poor white" in the strictest sense is a being beyond the pale of even the most generous democratic recognition; in the negro's term, "po' white trash," or so much social débris. [Robert Penn Warren, "The Briar Patch," 1930, footnote]

"day on which a memorial is made," by 1819, of any anniversary date, especially a religious anniversary; see memorial (adj.). As a specific end-of-May holiday commemorating U.S. war dead, it began informally in the late 1860s and originally commemorated the Northern soldiers killed in the Civil War. It was officially so called by 1869 among veterans' organizations, but Decoration Day also was used. The Grand Army of the Republic, the main veterans' organization in the North, officially designated it Memorial Day by resolution in 1882:

That the Commander-in-Chief be requested to issue a General Order calling the attention of the officers and members of the Grand Army of the Republic, and of the people at large, to the fact that the proper designation of May 30th is Memorial Day and to request that it may be always so called. [Grand Army Blue Book, Philadelphia, 1884]

The South, however, had its own Confederate Memorial Day, and there was some grumbling about the apparent appropriation of the name.

The word "Memorial" was adopted by the Maryland Confederates shortly after the war, and has been generally used throughout the South. It is distinctively Confederate in its origin and use, and I would suggest to all Confederate societies to adhere to it. The Federals' annual day of observance is known as "Decoration Day," having been made so by an act of Congress, and the 30th day of May named as the date. In Maryland there is annually a Decoration Day and a Memorial Day. The two words are expressive not only of the nature of the observance, but also of the people who participate therein. [Confederate Veteran, November 1893]

"battles among fellow citizens or within a community," from civil in a sense of "occurring among fellow citizens" attested from late 14c. in batayle ciuile "civil battle," etc. The exact phrase civil war is attested from late 15c. (the Latin phrase was bella civicus). An Old English word for it was ingewinn. Ancient Greek had polemos epidemios.

Early use typically was in reference to ancient Rome. Later, in England, to the struggle between Parliament and Charles I (1641-1651); in U.S., to the War of Secession (1861-1865), an application often decried as wholly inaccurate but in use (among other names) in the North during the war and boosted by the popular "Battles and Leaders of the Civil War" series published 1884-87 in "Century Magazine."

"The war between the States," which a good many Southerners prefer, is both bookish and inexact. "Civil war" is an utter misnomer. It was used and is still used by courteous people, the same people who are careful to say "Federal" and "Confederate." "War of the rebellion," which begs the very question at issue, has become the official designation of the struggle, but has found no acceptance with the vanquished. To this day no Southerner uses it except by way of quotation .... "The war of secession" is still used a good deal in foreign books, but it has no popular hold. "The war," without any further qualification, served the turn of Thucydides and Aristophanes for the Peloponnesian war. It will serve ours, let it be hoped, for some time to come. [Basil L. Gildersleeve, "The Creed of the Old South," 1915]

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 or influenced by 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, with sense as in the German phrase. Etymology sources seem to lean toward the German origin. The adverbial so long "for such a long time" is from late Old English (swa lange); see so.

Earlier guesses that it was a sailors' corruption of a South Pacific form of Arabic salaam are not now 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. The 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 expression is now (1923) often used by the literary and artistic classes.