Lometa does a much better job than I would at explaining the derivation of the term 'flak' in a writeup down below; it remains to me only to note that the term itself was originally spelt 'FlaK' and the acronym was used to designate various models of German anti-aircraft artillery (triple-A) pieces.

The most famous of these was probably the FlaK-18, or 'German 88,' an 88-millimeter version first introduced during the Spanish Civil War. It was during that war that the Wehrmacht discovered that the 88, due to its large shell, high muzzle velocity and consequent flat trajectory, was also quite useful for killing tanks - a factoid that was dutifully filed away in the Wehrmacht's institutional memory, as a directive that all 88mm FlaK units be issued solid shot for tank killing 'just in case.' These popped up again (to the dismay of the Allies) in the Second World War. In that war, a British and French combined armor force broke through a German Panzer group and its accompanying infantry units; the only weapons available to cobble together a defense were the anti-aircraft units sited farther back from the front. General Erwin Rommel, in charge of the sector, did just that, and the 88s were used with decisive success against British Matilda tanks which had just withstood everything the ground-bound German units had tossed at them. The attack was stopped.

    The African sun, like a bloody curious eye, hung on the rim of the world as hundreds of airplane engines coughed into life, spewing miniature dust storms across the flat wastes of a desert airfield.

    Thin aluminum skins of C-47s vibrated like drawn snare drums and as paratroopers heaved themselves up into the planes and sought their predesignated seats, they wrinkled their noses at the smell of gasoline and lacquer that flooded the planes' interiors.

    Spearheading the airborne invasion of Sicily, the 3rd Battalion, 504th Parachute Infantry, crossed the North African coast as the sun flared briefly, then plummeted into the Mediterranean. Flak rose thinly into the dusky sky ahead-probably Malta, the paratroopers grimly thought.

    Detached from the regiment for tactical requirements, the 3rd Battalion crossed over the Sicilian coast on schedule and jumped on its assigned drop zone, July 9, 1943 - the first Allied troops to land in the invasion that Prime Minister Winston Churchill termed, "not the beginning of the end, but the end of the beginning".

    from a personal tribute to the 504th Parachute Infantry Brigade

Pronounced 'flak this noun is as mentioned above as an acronym for anti-aircraft artillery,also refers to their bursting shells Here is some more information as to what historically motivated the word.

Like blitz, dachshund and dollar, flak is a German loanword first coined in 1938 for Fliegerabwehrkanone as in Flieger+Abwehr+Kanone for “flyer defense gun,” or more literally a "pilot-defense-gun.” The military meaning was obtained from "anti-aircraft fire" in 1940.

During World War II the British called flak ack-ack or Archie. "Ack" was then the British symbolic enunciation for "a" so "ack-ack" stood for a-a or anti-aircraft. By December 1942 "ack" was replaced by "able" as the symbolic enunciation of "a" in the military system of symbolic letter substitutes.

The Automatic 2 cm Anti-Aircraft Gun was initially used successfully in the Spanish Civil War in 1936 during the battle of Bilabao. Earlier the weapon had been introduced into the German Luftwaffe in 1934 as a replacement for the anti-aircraft machine-gun MG C30. The purpose was to provide a defense against low flying aircrafts. As a German defense system flak units employed more than a million men and women and POW’s during the war, were responsible for more than half of all Allied aircraft losses, forced Allied bombers to fly far above high-accuracy altitudes, and permitted Germany to hold out far longer than it might have otherwise. Hitler was obsessed with micromanaging the war. Nearing the target's searchlights, sometimes guided by primitive radar the flak unit would hunt for a plane by attempting to cone it between two different beams of light to as a target for the anti-aircraft guns.

    The searchlights were fantastic, they really were. They were worse really than flak because you didn't see that until it actually burst. But to fly towards a target which was literally ringed with cones of searchlights, with the flak going up into where all the beams met at a focal point, and then all the guns round it would concentrate on that area once they got someone in it. It was quite scary.
Flak was intended to explode at the same altitude as the planes, throwing out shards of hot metal that easily tore through the thin skins of the bombers. The 'tail-end Charlie’ or rear gunner was exposed to the below freezing night air. Frequently he would knock out a panel from the gun turret to increase the opportunity of spotting German fighters. Crews were acutely aware of their vulnerability in a plane laden with bombs and fuel:
    “You had to literally fly through a wall of flak..... often getting chunks of metal come pinging in to the aircraft. And sometimes you could smell the cordite in the aircraft.” -Tom Wingham
The war forced many innovations in radar, navigation aids and bomb sites. Some were cleverly successful. The Russian women pilots developed a clever strategy for dealing with German flak traps:
    The women of the 588th (Women's Bomber Regiment) also came up with a unique strategy to deal with the German "flak circuses." In an area they controlled or were protecting, the Germans would assemble as many as two dozen 37 mm antiaircraft guns in concentric circles around a target. The gunners would be supported by a searchlight platoon. Many Soviet bomber pilots, true to their unyielding strategy, flew straight in, alit by searchlights and pounded by ring after ring of antiaircraft fire; they seldom made it to the target. After several casualties among the women bomber pilots, the survivors decided to break out of the Soviet strategy mold. The women decided to fly in groups of three instead of two; two would fly headlong toward the target while one held back. When the front two were first hit by the searchlights, they would go into wild evasive action, and the searchlight operators would try to follow them. The third plane would slip in under their cover and deliver her load. The women would later describe the tension of waiting for the searchlight to hit them, how they ignored the sound of flak tearing through the wings and fuselage, and their worry about their friends who had been sent in to draw the Germans' fire. But, in the end, they seemed to shrug the tactic off in a pragmatic military manner. "It worked," they said.
Another tactic to deal with “flak alley” by the British was a raid codenamed Window. Above the skies in Hamburg in July 1943 British planes dropped thousands of strips of aluminum foil.
    The German Flak was aimed visually and by radar. In the thirty or so miles to the target we used electronic jamming and dispensed chaff. The chaff looked like the old time X-mas tree tinsel. We used those items to fool their radar, but if it was a clear day they used visual sighting and that negated our counter measures. (Cubbins, William R., The War of the Cottontails, p. 45)
Fluttering down the strips created a mass of reflections on German radar screens making it almost impossible to distinguish the bombers. The Germans developed a better radar system as a result. There were flak traps, flak towers and flak fighting was even taking to sea as U boats escorted flak boats and the resulting flak trawlers who opposed them. By the end of 1944 US Army Air Force fighter planes controlled the skies over Europe. Most of the flak batteries had been neutralized and were retreating from the Russian Armies; withdrawing their guns the Germans used them to strengthen their defenses closer to home.

Flak related phrases used by American airmen from 1943-1945 were, 'flak farm,' the 'flak shack' and a 'flak home,' which led to more terms like, "flak-happy," a person mentally affected by flak. One B-26 nick named Flak Bait, a famous Marauder, endured over 1000 enemy hits during her combat days. Today the un-restored nose section is displayed at the National Air and Space Museum of the Smithsonian Institution in Washington D.C.

The word has been extended to flak jacket, slang for a bullet-proof vest. By 1946, the word "flack" meant a press agent when it was first used by show biz magazine Variety, supposedly from name of Gene Flack, a movie agent. Along with this change in spelling, there seems to be a semantic shift from "flak" as a defensive to an offensive sense leading to the idiom “flak catcher,” “ a person who takes the heat” or "a slick spokesperson who can turn any criticism to the advantage of their employer." Sometime around 1963 it became an American colloquialism for opposition or heavy criticism. which could lead some to think of a flak catcher as a limited definition of a public relations agent and that reporters could fire “flak at a flack.”

Sources:

BBC - History - World War Two - The Air War and British Bomber:
www.bbc.co.uk/history/war/wwtwo/bomber3.shtml

Dr. Link Answers your Questions:
www.poynter.org/dr_ink/december2000.htm

etymology:
www.geocities.com/etymonline/f3etym.htm

Women's History Month Feature:
www.faa.gov/avr/news/Nexen.htm

yourdictionary.com:
http://www.yourdictionary.com/

While flak rarely shot down many enemy planes, it had a dramatic effect on the effectiveness of air attacks.

Anti-aircraft guns can be divided into two major categories: rapid-fire, low-caliber guns such as quad guns, and slower-firing, high-caliber guns such as the '88'. The former type is still in use today, while the latter type has been largely replaced by SAMs.

'Light flak', usually in .50, 20mm, or 30mm caliber, could not reach very high into the sky. However, it was very effective at destroying low, slow-flying planes within its reach. This typically forced WWII bomber pilots to fly higher, so as to remain out of range of the light flak. Since the bombsights of that era were crude at best, this enormously degraded bombing accuracy. It also disrupted attempts to use low-level bombing attacks against troops in the field.

'Heavy flak', such as the famed 88mm gun, on the other hand, could lob shells to altitudes as high as a bomber could fly. The difficulties of getting a targeting solution against a bomber several kilometers in the air were formidable, though, and few aircraft were downed by flak hits. The flak still interfered with bombing operations despite this. Bombers under flak attack tended to take evasive action, increasing the chance of navigational errors during flight and of misses during the bomb run itself. Also, many aircraft were damaged by flak. This could make them easier prey for fighters, possibly damage them enough that they would be forced to crash land before returning to base, and force the bombers' ground crew to repair them before sending them on another sortie.

On the other hand, the outlay required to maintain this anti-aircraft force severely harmed the German war effort. Roughly one third of all artillery production was dedicated to manufacturing antiaircraft guns and ammunition, most of which were used for home defense. An even larger proportion of Germany's optics and electronics industries were dedicated to the flak batteries. If most of those resources had instead been devoted to the fighting front, it is possible that the Germans would have won the war before Allied strategic bombing attacks became powerful enough to be ptentially decisive.

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