In "Three-Day Blow," what do the following lines mean: "What did the Cards do? Dropped a double header to the Giants. That ought to cinch it for them."
In this story, Nick and Bill are very interested in the sport of baseball. This line that you cite has to do with baseball.
The Cards are a baseball team. The name is short for "Cardinals." They play in St. Louis, Missouri. The Giants are another team. At the time this book was written they played in New York City.
A double header is when baseball teams played two games against each other on the same day. This used to happen a lot but no longer does. The Cards must have lost both games.
That ought to "cinch" it for the Giants -- that means by winning those two games they will guarantee that they will win the league championship.