I used the expression for good recently, in the standard way, meaning forever. And then I thought: why do we use for good in this way?
It doesn’t really seem very logical. I thought about it for a while, and then looked it up. Which cleared things up a little, at least.
According to the Oxford English Dictionary, the expression is most likely derived from for good and for all, from at least the 16th century. For all in this case meant for all time (like once and for all), so it makes sense that the expression, becoming shorter over time and dropping …and for all, would still keep the meaning of forever.
I think there’s a little more to it than that though. I think it’s significant that we’ve kept the for good part, and not for all. The meaning of the original expression was as a valid conclusion, and I think that there’s still some of that positive sense in the expression. When we use it, it’s often with a positive meaning. If we tell someone we’re back for good, for example, we generally expect them to be happy at that news.
And that’s that settled.
For good.
English-Language Thoughts 
Looking forward to the next in the series about Take That songs.
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In-depth post-structuralist analysis of Relight my Fire incoming…
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I look forward to it. Hopefully you Never Forget
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Maybe it’s one of those phrases (the original and subsequent omission) that possibly encompass humanity. “For good” express that optimism that some are reluctant to entertain, yet there it is.
“For all” isn’t compatible with the … I would be bold as to say the more recent and the general consensus that there isn’t any “for all”; nothing is forever, so let’s not bother with that bit and focus on what we can work with. I hope this makes sense, I’m caffeine deprived at the moment.
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I think that makes a lot of sense, and some people have suggested that ‘for all meant ‘for all people.’
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