'ack-ack' originates from early 20th-century English radio/telephony spelling, specifically the word 'Ack' used for the letter 'A'; the doubled form 'Ack-Ack' was used to represent 'A A' (for 'anti-aircraft').

'Ack' (a spelling-alphabet word for the letter A) was used in early British signal/telephony alphabets; during World War I and World War II the repetition 'Ack-Ack' (A A) came to be used as a slang label for anti-aircraft guns and their fire, and later solidified in English as 'ack-ack'.

Initially a spoken/written representation of the letter 'A' ('Ack'), it evolved into slang referring specifically to anti-aircraft guns or their fire and retained that specialized military meaning.