So Long comes from the ghettoes of New York where irish,italian Jewish and Arab immigrants mixed together. Jews and Arabs always greet and say goodbye to each other by saying Shalom,Shalom, or Salaam,Salaam. These words were corrupted into solong, solong. Ultimately folks believed that it meant that it had been so long since you had seen someone. Not so.
So long Bobby.See you tomorra.
so long has the same meaning of BYE dude. used by cowboys often.
a : so long cowboy.
b : take care.