Old English ald (Anglian), eald (West Saxon, Kentish) "antique, of ancient origin, belonging to antiquity, primeval; long in existence or use; near the end of the normal span of life; elder, mature, experienced," from Proto-Germanic *althaz "grown up, adult" (source also of Old Frisian ald, Gothic alþeis, Dutch oud, German alt), originally a past-participle stem of a verb meaning "grow, nourish" (compare Gothic alan "to grow up," Old Norse ala "to nourish"), from PIE root *al- (2) "to grow, nourish." The original Old English vowel is preserved in Scots auld, also in alderman. The original comparative and superlative (elder, eldest) are retained in particular uses.
The usual PIE root is *sen- (see senior (adj.)). A few Indo-European languages distinguish words for "old" (vs. young) from words for "old" (vs. new), and some have separate words for aged persons as opposed to old things. Latin senex was used of aged living things, mostly persons, while vetus (literally "having many years") was used of inanimate things. Greek geraios was used mostly of humans; palaios was used mostly of things, of persons only in a derogatory sense. Greek also had arkhaios, literally "belonging to the beginning," which parallels French ancien, used mostly with reference to things "of former times."
Old English also had fyrn "ancient," which is related to Old English feor "far, distant" (see far, and compare Gothic fairneis, Old Norse forn "old, of old, of former times," Old High German firni "old, experienced").
Meaning "of a specified age" (three days old) is from late Old English. Sense of "pertaining to or characteristic of the earlier or earliest of two or more stages of development or periods of time" is from late Old English. As an intensive, "great, high," mid-15c., now only following another adjective (gay old time, good old Charlie Brown). As a noun, "those who are old," 12c. Of old "of old times" is from late 14c.
Old age "period of life of advanced years" is from early 14c. Old Testament is attested from mid-14c. (in late Old English it was old law). Old lady "wife, mother" is attested from c. 1775 (but compare Old English seo ealde hlæfdige "the queen dowager"). Old man "man who has lived long" is from late Old English; the sense of "husband, father, boss" is from 1854, earlier (1830) it was military slang for "commanding officer;" old boy as a familiar form of address is by c. 1600. Old days "former times" is from late Old English; good old days, "former times conceived as better than the present," sometimes ironic, is by 1670s. Old Light (adj.), in religion, "favoring the old faith or principles," is by 1819.
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.