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.