1. n. complaints; criticism; negative feedback. (Originally referred to antiaircraft guns and the explosions and damage they caused. The first form is an initialism from German Fliegerabwehrkanonen = flyer defense cannons. I.e., the initial fl plus the first a plus the k.) Why do I have to get all the flak for what you did?
2. n. publicity; hype. Who is going to believe this flack about being first-rate?
3. n. a public relations agent or officer. The flak made an announcement and then disappeared.