What Does So Long Mean

What does the phrase so long mean?

‘So Long’: A Common But Mysterious Goodbye

Its origins are surprisingly murky. So long, an expression of farewell, is a colloquialism that we take for granted as being a logical construction. … Some believe the phrase to be adapted from another language.

Is it rude to say so long?

“So long” sounds a lot less dated, and conveys equally well the sense of finality that you were looking for, kwyjibo. I’d say “So long” is the better option. However, it would almost always be rude to say either of these to anyone.