“Flak” was World War II airmens’ lingo for anti-aircraft gunfire. The term was a telescoping of the German word “Flugabwehrkanone,” which means “aircraft defense cannon.” Rather than the cannon, airmen used “flak” to refer to the puffs of black or white smoke from the exploding ammunition that filled the skies and posed the greatest threat to their lives.
The photo at right shows a German 88mm anti-aircraft gun of the type that would have been used against Allied aircraft such as Yossarian's B-25. The photo comes from the Deutches Bundesarchiv, or German Federal Archive, and dates from 1943.
Below, a U.S. consolidated B-24 escapes from an area of the sky spotted with flak, but with its no. 2 engine smoking.