I wonder why 'for good' mean 'forever'

Question

I've been asked "will you stay in this country for good?"

At that time I didn't know "for good" mean forever.

I wonder why it could be forever.

Answer 1

Since the 1300s, among the meanings of good, when used of things that could be counted or measured (like time) has been "quite long, large, many" and then "all" or "the entirety" or "completely".

The idiom "for good" (meaning "for a very long time" or "for all time") ultimately came to mean "forever" or "for the rest of your days".

So, today, when we say "He's here for good" we mean "He's here to stay. He is not going to leave".

Answer 2

This relates to the "Bigger is Better" perspective/metaphor. Thus more of something is better than less (a good amount; a goodly amount). This applies to time as well (a good amount of time; a goodly amount of time).

Also consider the line from the wedding service: for richer for poorer, ... This idea of for better or worse, for good or ill, can lose the second part. You are (were) committing for life (in an era where marriage was a contract and divorce a rarity). I would see (and I've seen this suggested) the idiom as primarily an abbreviated form of "for good or ill".

Furthermore, the assumption and associated logic around "for good" in the temporal sense is that the move (change of state) is good and so indefinite continuation is good (and a further change of state is bad and would only be made if things didn't turn out as expected).

Another linguistic idea is that completion = perfection = good (you have attained your goal). Conversely, it is not good to be making yet another move/change in your circumstances.

Generally, while one of these answers might reflect the true origins, all seem to contribute to the current range of sense and usage of "for good".