Anti-aircraft gunfire, or strong criticism and disapproval.
Acronym from German 'Flugabwehrkanone' (anti-aircraft gun). It was borrowed into English during World War II, then evolved figuratively to mean criticism that 'attacks' you.
'Flak' is a German military term that English borrowed during WWII when it was literally dangerous—flak burst in the air near planes. After the war, people started using it metaphorically for any hostile criticism, like 'getting flak for your decision.'
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