Why does "for good" mean "forever"?

It may be the for all that originally carried most of the 'forever' meaning, as in the expression once and for all, which covers pretty nearly the same ground. If so, the good part may have been largely an intensifier, with a sense somewhat akin to that of good in a good handful, a good while, etc. However, the relevant entry in the Middle English Dictionary notes by gode inspeccioun 'by thorough inspection', in hys good lyve 'during his whole life', and gode journeie 'a full day’s journey', in which good has the sense 'full, entire, complete'. This is an even better fit with the conjecture, but it could also suggest that for good by itself might have carried some of the same sense as for all -- the 'completely' part, if not the 'forever' part. In this connection it may be significant that Dutch voorgoed, literally 'for-good', has about the same range of senses as for good (and all); unfortunately, I’ve not discovered anything about its history.

The context is "for good and all" time. Which means "forever."