What's the meaning of the phrase 'Ack-ack'?
'Ack-ack' is a the name of anti-aircraft gunfire. It has also been used as the name of an anti-aircraft gun and the regiment that used it.
What's the origin of the phrase 'Ack-ack'?
Military forces have used ways of signalling to each other for centuries. The use of flags was one way of communicating. One of the most famous lines in British naval history is Admiral Nelson's "England expects that every man will do his duty" was spelled out in flags.
In the late 19th century field telephones superseded flags. These were first used by the British in the Second Boer War in 1899. The poor quality lines on the early telephones meant that letters like B, T, C etc. were easily confused. In anticipation of the use in the war the British War Office issued a phonetic alphabet in 1898 called The Signalling Instructions:
The letters T, A, B, M,..will be called toc, ak, beer, emma.,,
The letter relevant to us here is A = ak.
Anti-aircraft fire was shortened to AA which, when spoken over a field telephone, became ak-ak.
The extended 1904 version of the Signalling Instructions gave A as ack, which gives us our present spelling of ack-ack.
The first time I know of ack-ack being used in print is in this explanation given in the English newspaper The Daily Mirror in August 1916:
The ‘Ack-Ack’ gun... is just an anti-aircraft gun, ‘Ack’ being the name for ‘A’.
The phonetic language we are more familiar with now is the Nato Phonetic Alphabet. In that A is alpha. If that had been adopted first I doubt that the guns would ever have been called Alpha-alpha guns.