I read “..gone for good.” somewhere this morning and it occurred to me that this might be an expression that was quite old. What does the OED say about it?

OED:

for good (and all): as a valid conclusion; hence, as a final act, finally.
15.. Parl. Byrdes Aij, Than desyred al the Byrdes great and smal to mewe the hauke for good and all. 1603 in Crt. & Times Jas. I (1849) I. 25 D’Auval.. is gone for good and all. 1687 CONGREVE Old Bach. I. i, Ay, you may take him for good-and-all if you will. 1711 SWIFT Jrnl. to Stella 4 July, This day I left Chelsea for good, (that’s a genteel phrase). a1732 T. BOSTON Crook in Lot (1805) 37 He was obliged for good and all to leave his country. 1850 J. H. NEWMAN Diffic. Anglic. 324 Throw off, for good and all, the illusions of your intellect. 1882 W. E. FORSTER Let. To Gladstone 10 Apr. in T. W. Reid Life (1888) II. viii. 421 This morning we released Parnell—not for good, but on parole.

I’m still baffled by the etymology of the phrase, why “for good” should mean “for ever”.

I agree with bayard on this.  However, of the idiomatic forms built on the word good, few make sense.  AHD has them as:

as good as Practically; nearly: as good as new.

no good Informal 1. Worthless. 2. Futile; useless: It’s no good arguing with them.

to the good 1. For the best; advantageous. 2. In an advantageous financial position: ended up to the good.

but good Informal Used as an intensive: The pipe started to leak but good.

good and Informal Very; thoroughly: I’ll do it when I’m good and ready.

for good Permanently; forever: I’m moving to Europe for good.

The first three (I’ve regrouped them) make a kind of literal sense.  The last three (including the phrase under consideration here) do not.

Certain not-for-profit agencies in my Nothern US rust-belt city are using the phrase “In the City for Good” playing on its literal and idiomatic readings.  We’re in the city doing good and we’re not leaving.

Interesting as I had assumed that ‘for good and all’ was a modern colloquialism or localism and am amazed to find the long version is as old as it is

Since it is as old as that, I wonder whether the Indo-European root is a clue to what we now perceive as the idiom.

Entry: ghedh- To unite, join, fit.

Together and gather seem to share this root, if AHD and Pokorny are right.

In Dutch it is ‘voorgoed’ with the same meaning. WNT gives two meanings.
As indication of degree: state of being as is, as opposed to a previous situation of uncertainty, now certain and without doubt.
As indication of time: indicating a future validity; unlimited, eternal, unchanged, etc.

It seems to me that ‘goed/good’ could also indicate something like ‘complete, fitting, of the highest value’ and similar (’gold’ is related as well). Maybe ‘for good’ is an extension of that.

I’m still baffled by the etymology of the phrase, why “for good” should mean “for ever”.

I too am still baffled.

I might be wrong, but “for good and all” sounds like “for good, bad, or for no particular reason”. If that is the case, it makes sense. When the “...and all” is dropped, it makes less sense.