Green’s Dictionary of Slang

(orig. mainly US) goodbye.

1848		‘Ned Buntline’ Mysteries and Miseries of N.Y. III 42: So long, till to-morrow night, old gal!
1854		E.T. Perkins Na Motu 55: And let us be friends again ere exchanging ‘So long’ forever.
1874		‘A.P.’ [Arthur Pember] Mysteries and Miseries 410: [H]e exclaimed, ‘S’long,’ and swayed away up the street.
1884		C.F. Lummis letter 13 Nov. in Byrkit Letters from the Southwest (1989) 92: Till then, in the language of the plains, ‘so long’.
1887		H. Frederic Seth’s Brother’s Wife 108: So long, boys!
1893		H. Lawson ‘An Incident at Stiffner’s’ in Roderick (1972) 117: He chucked his saddle onto his horse, chucked himself into the saddle, said ‘s’long,’ and slithered off.
1902		J. Masefield ‘A Valediction’ in Salt-Water Ballads 37: Well, so-long, Billy, ’n’ a spankin’ heavy pay-day to you!
1910		H.G. Wells Hist. of Mr Polly (1946) 175: ‘So long,’ said Mr. Polly.
1914		Sun. Times (Perth) 1 Mar. 8s/7: ‘Then [...] yer can shout for them yourself. S’long.’ And he S’longed!
1925		S. O’Casey Shadow of a Gunman Act I: I must be off too; so long, Mr. Davoren.
1934		J. Franklyn This Gutter Life 103: ‘So long!’ Dicky opened the door and shot away like an arrow.
1942		A. Christie Body in the Library (1959) 143: So long, Dinah.
1953		B. Stronach Musterer on Molesworth 69: ‘So long,’ said I.
1963		A.S. Fleischman Venetian Blonde (2006) 149: ‘Ciao?’ ‘Chow.’ ‘So long.’ ‘So long.’ ‘Goodbye.’.
1978		L. Kramer Faggots 267: ‘So long, Feff.’ This time I’m leaving you.
1989		C. Hiaasen Skin Tight 252: Took off [...] Gobbled three Darvocets and said so long.
1997		(con. 1950s) J. Byrne Slab Boys [film script] 97: So long, Slab Boys.
2015		S. King Finders Keepers (2016) 158: Hodge flashes a so-long with his lights.