Answer:
A Flak Cannon is an anti-aircraft gun. The word flak comes from the German word Fliegerabwehrkanone, aircraft defence cannon. They were created before World War One, and used throughout the century. These are also known as AA guns, in English (anti-aircraft). If you've every seen old footage from a past war, the guns mounted on the front of battleships that have two large cannons that fire one after each other: FLAK Cannon. They shoot pretty much cellphone-sized bullets that tear through the metal of the airplanes.