Flack is a military defense, a special gun that shoots into the sky, defending against air attacks. During many battles in World War II, the sound of flack filled the air.
Anti-aircraft guns are flack. also spelled flak. This military definition gave rise to another meaning of flack, criticism that's so fierce that it feels like an attack: "Politicians have to be strong enough to take a lot of flack from their critics." You can also use the word to mean a promoter or salesman, especially one who is especially savvy or smooth-talking. The German root is Fliegerabwehrkanone, "aviator-defense gun."