What's the origin of this strange substitute for farewell? We say it all the time, but I can't figure out its meaning.
There is a Gaelic (Irish) word for goodbye, it is pronounced, roughly, 'slahng.' I expect it would have been very easy to have been brought into American use during the wave of Irish immigration in the 1800's, and for Americans to misinterpret it as the two English words, 'so long.'
"Slán" is the most common way to say goodbye in Irish. I always thought "So long" comes from it. Here's how it's pronounced in each of the three dialects of Irish:
It's probably one of the general category of 'expectant' goodbyes that mean something like 'good wishes until we next see each other' - i.e. 'For so long (i.e. however long) it takes until we meet again'. Compare German so lange or bis bald or French au revoir.