An anti-aircraft gun was first called an Ack-Ack gun during WWI because A in the military alphabet at that time was Ack in the same way as A is Alfa today in the widely accepted NATO alphabet. Thus, the acronym for anti-aircraft is AA, which translated into the military alphabet used in 1914-18 was Ack-Ack. The military alphabet during WWI started with Ack, Beer and Charlie. During WWII, it became Able, Baker and Charlie and today in the NATO Alphabet, which was adopted internationally in the 1950s, we have Alfa, Bravo, Charlie. Therefore, dear old Charlie has remained constant throughout.