A very recent and similar question was closed asks what "for good" means. While general reference can answer the question, I became curious as to the etymology of the idiom. Googling around got me nowhere, which is unusual. The closest I found was a thread in wordorigins.org:
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). a 1732 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.
However, this only traces some historical usages, and suggests that for good and all used to be a more popular version of the current idiom. Is there anything more substantial? Why did "for good" come to mean "forever"?