The Online Etymology Dictionary entry on
so long says, in part:
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, Mor’n 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 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.
