'flak' originates from German, specifically the abbreviation 'Flak' from 'Flugabwehrkanone', where 'Flug' meant 'flight', 'Abwehr' meant 'defense', and 'Kanone' meant 'cannon'.

'flak' was borrowed into English in the 20th century (especially during and after World War I/II) from German 'Flak' (an abbreviation of 'Flugabwehrkanone') and entered English initially as a military term referring to anti-aircraft fire; it later developed figurative senses.

Initially it meant 'anti-aircraft fire or the guns/shells used against aircraft', but over time it also evolved into the figurative sense of 'severe criticism' or 'hostile reaction'.