so long

noun Informal.
good-bye: I said so long and left.

Origin:
1840–50, Americanism

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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Collins
World English Dictionary
so long
 
sentence substitute
1.  informal farewell; goodbye
 
adv
2.  slang (South African) for the time being; meanwhile

Collins English Dictionary - Complete & Unabridged 10th Edition
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Etymonline
Word Origin & History

so long
parting salutation, 1860, of unknown origin, perhaps from a Ger. idiom (cf. Ger. parting salutation adieu so lange, the full sense of which probably is something like "farewell, whilst (we're apart)"), perhaps from Heb. shalom (via Yiddish sholom). Some have noted a similarity to Scand. leave-taking
phrases, cf. Norw. Adjø så lenge, Farvel så lenge, Morn så lenge, lit. "bye so long, farewell so long, morning so long;" and Swed. Hej så länge "good-bye for now," with så länge "for now" attested since 1850 according to Swed. sources. Most etymology sources seem to lean toward the Ger. origin. 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 Ir. slan "health," said to be used as a toast and a salutation. The phrase seems to have turned up simultaneously in Amer.Eng., 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 dreamd, more direct, darts awakening rays about meSo long!
Remember my wordsI may again return,
I love youI depart from materials;
I am as one disembodied, triumphant, dead.
Whitman's friend and fan William Sloane Kennedy, wrote in 1923:
"The salutation of partingSo 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, & prostitutesthe 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."
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Idioms & Phrases

so long

Good-bye, as in So long, we'll see you next week. The allusion here is puzzling; long presumably means "a long time" and perhaps the sense is "until we meet again after a long time," but the usage has no such implication. [Colloquial; first half of 1800s]

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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Example sentences
Your kids can only bob in the water for so long before the chlorine turns their
  hair a luminescent green.
One of the problems with neuropsychiatric drugs is that they take so long to
  develop.
Please accept my apologies for letting this blog go so long without an update.
It took so long to find because it's an extremely dim and cool kind of failed
  star called a brown dwarf.
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