I have heard this a few years ago, meaning someone is in very good shape, very active, very fit.
But I wonder if it is still said and which connotation this phrase has.
German Language Stack Exchange is a bilingual question and answer site for speakers of all levels who want to share and increase their knowledge of the German language. It's 100% free, no registration required.Sign up to join this community
It's an expression of admiration and has a somewhat old feeling to it. I would mostly associate it with older people who managed to keep particularly fit despite their age.
It's perfectly fine to use this in everyday conversations, but it may sound a little bit patronizing/condescending if the person mentioned doesn't feel as old yet.
An example would be
Der Mann ist gut in Schwung, er ist 70 Jahre alt und läuft einen Marathon.