What is the origin of "So long"?
Solution 1:
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.'
Solution 2:
"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:
Irish:http://www.teanglann.ie/ga/fuaim/Slán