Flak

google

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1930s (in flak (sense 2)): from German, abbreviation of Fliegerabwehrkanone, literally ‘aviator-defence gun’.

wiktionary

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Borrowed from German FlaK, short for Fliegerabwehrkanone(“anti aeroplane cannon”).

etymonline

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flak (n.)

1938, "anti-aircraft gun," from German Flak, condensed from Fliegerabwehrkanone, literally "pilot warding-off cannon." Sense of "anti-aircraft fire" is from 1940; metaphoric sense of "criticism" is c. 1963 in American English. Flak jacket is by 1956.