1
general::
Phrase(s): So long.
Good-bye. • So long, see ya later. • It’s been good talking to you. So long.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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]
American Heritage Idioms