Douglas Harper's Etymology Dictionary
c.1500, Scottish variant of long since; popularized in Burns' song, 1788.
Wiktionary
adv. long ago
WordNet
adv. of the distant or comparatively distant past; "We met once long ago"; "they long ago forsook their nomadic life"; "left for work long ago"; "he has long since given up mountain climbing"; "This name has long since been forgotten"; "lang syne" is Scottish [syn: long ago, long since]
Usage examples of "lang syne".
Squeer: here now we are the four of us: old Matt Gregory and old Marcus and old Luke Tarpey: the four of us and sure, thank God, there are no more of us: and, sure now, you wouldn't go and forget and leave out the other fellow and old Johnny MacDougall: the four of us and no more of us and so now pass the fish for Christ sake, Amen: the way they used to be saying their grace before fish, repeating itself, after the interims of Augusburgh for auld lang syne.
Thus we came round the clock, upon the Great North Road, to the performance of Auld Lang Syne by day again.
And I think we both know that he's in a business where he can't use Auld Lang Syne' as a personnel selection criterion.
They banged their way through endless repetitions of `Edelweiss', `Greensleeves' and 'Auld Lang Syne' interspersed with the occasional bash at `New York, New York', 'Chicago' and 'I Left My Heart in San Francisco'.