In the sentence, what does for good mean?
When he left, he was extremely disappointed. I think he’s gone for good.
Note
permanently is correct because for good means permanently or forever.
This idiom or phrase is useful because it helps students understand the intended meaning beyond the individual words.
Examples: She moved to Canada for good last year. After the final warning, the shop closed for good.
Here good does not mean moral goodness; the phrase means permanently.