Origin of the word "so long"

I want to know the origin of the word "so long" (as used to mean goodbye) in the English Language

Used as a good-bye? I think it's from Shalom. Do you know how to look up stuff on Google?

Try Shalom so long...

That'll get you one man's opinion, from the Woody Guthrie song. So long...

Woody Guthrie dustbowl song, So Long, It's Been Good to Know You. The refrain goes,

So long, it's been good to know you,
So long, it's been good to know you,
So long, it's been good to know you,
But this dusty old dust is getting me down,
And I've got to be moving along.

There are some theories here:

http://www.m-w.com/wftw/00apr/040600.htm

I'm surprised that MW didn't specifically mention so long used as a legal time frame. It's used a lot as a phrase in documents... "the taxes will be paid so long as the building is occupied" for example.

From there, it seems to me a hop & skip to "so long (as we're apart) and a wave <goodbye>."

Origins of the phrase 'so long'
I think it quite likely that this is corrupted Irish. The phrase 'Slan go fuill' can sound very much like 'so long (fool)'.

Slan go = so long

It's only an idea, but it's a plausible one. The English expression for saying something is very good - 'that's SMASHING' - is straight from Gaelic, and such absorptions from Irish immigrants is not unlikely.

Is mise,

Alan.