For good means permanently, with no going back. This is the last time.
It always follows the verb, and it attaches to leaving, stopping and closing, so a shop closing for good is not reopening. The phrase is short and final, which is why it lands harder than permanently. There is a longer version, for good and all, which is old-fashioned and emphatic. It has nothing to do with goodness, and the good here is an old noun meaning benefit or finality.
1phrasefor goodPermanently and finally.
“He left the country for good in 1998.”
From for + good.
Etymology from Wiktionary, CC BY-SA
Practised in Idioms, A Man Called Ove
First appears in chapter 6 of A Man Called Ove.
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