There are a number of German words that came into English during the two World Wars. Flak is one of them.
Flak came into English in the 1930s and originally referred to anti-aircraft guns, and then later to anti-aircraft fire, and especially the bursting shells of anti-aircraft fire. That later sense gave rise to flak jacket, or a jacket designed to protect the wearer from injury from flak, shrapnel , or bullets. In the 1960s, flak gained a much broader use: it came to refer to any sudden criticism. It shows up in sentences that still mirror its military flavor, like “The company took some flak from unhappy investors.”
Flak is a direct borrowing from German, where it was actually an acronym. In German, new words can be created simply by smooshing together existing words, and this can lead to some really unwieldy compounds. (Mark Twain wrote, “Some German words are so long that they have a perspective” in an appendix to A Tramp Abroad that was appropriately titled The Awful German Language). One such compound was the name for an anti-aircraft gun: Fliegerabwehrkanone. The word was a compound of the earlier Fliegerabwehr, which means “defense against air attack,” and Kanone, which means “cannon.” Fliegerabwehr itself is another compound made up of Flieger, “aviator,” and Abwehr, “defense.” While not terribly long by German standards, Fliegerabwehrkanone was awkward enough that when the guns were used in the field of war, the name was shortened to Flak, from Fliegerabwehrkanone.
Flak has been so far removed from its German origins that it’s often confused with another word, flack. The latter is used to refer to a press agent or spin doctor; it’s been so confused with flak that we now consider flack to be a lesser-used variant spelling of flak.