- antiaircraft fire, especially as experienced by the crews of combat airplanes at which the fire is directed.
- criticism; hostile reaction; abuse
1938, from Ger. Flak, acronym for Fliegerabwehrkanone "airplane defense cannon." Sense of "anti-aircraft fire" is 1940; metaphoric sense of "criticism" is c.1963 in Amer.Eng.
"Hamburg knew the bombs were coming, and so the prisoners of war and forced laborers had just half a year to build the giant flak bunker."
- Robert Kunzig, "The Will to Change", The Best American Science and Nature Writing 2016