Typically speaking in the aesthetic sense, _punk means taking a certain look to its extreme. Cyberpunk of course infusing everything with computer technology, steampunk infusing everything with the looks of a steam powered machine, etc.
Starfield was described once as having its aesthetic “NASApunk,” which sounded really cool to me when I heard it. I expected white and black, gold foil, etc. Which isn’t really how the end product ended which was a bit disappointing, but the point remained that calling it “NASApunk” had me immediately expecting a certain aesthetic.
In the case of Frostpunk, I am not sure. It takes place in a frozen world, but it doesn’t have an aesthetic to fit that name so it may just be a title.