So long

Do you ever wonder why we say, “So long”, when we are taking leave of someone? Where does the phrase ‘so long’ come from? In brief, no one knows.
 
The term ‘so long’ first appears in the 19th century and was widely used in the United States, Britain, and Canada, particularly among lower classes (Online Etymological Dictionary).
 
Numerous suggestions for the origins have been made. For example, the German idiom adieu so lange (farewell, while we’re apart). Or Hebrew shalom. Or Norwegian adjo sa lenge (bye so long). Or Swedish sa lange (for now). Or from garbled Arabic words in sailors’ slang. However, the Online Etymological Dictionary suggests that many etymological sources lean toward a Germanic origin.
 
However, the noted etymologist, Anatoly Liberman, states, “…there is no consensus on the origin of ‘so long’. Yet, despite all doubts, the idea that we are dealing with a sailor’s phrase seems right. If this idea is acceptable, ‘so long’ is, most probably, a garbled version of some foreign word.”
 
The Cambridge Dictionary suggests that we can use “So long” as a less formal way of saying “Goodbye” so long as we are not sure of when we’re returning. That is, we might say, “So long, catch up with you later” rather than “Goodbye, see you tomorrow.”
 
Anyway, so long for now.
 
Reference: Online Etymological Dictionary, https://www.etymonline.com/
https://blog.oup.com/2018/03/the-origin-of-so-long/
https://dictionary.cambridge.org/grammar/british-grammar/as-long-as-and-so-long-as
​