Word Study
ack-ack
CIDE DICTIONARY
ack-ack, n. [from AA, meaning antiaircraft pronounced ack-ack by British signalmen.].
Artillery designed to shoot upward at airplanes; antiaircraft artillery. [WordNet 1.5]
OXFORD DICTIONARY
ack-ack, adj. & n. colloq.
--adj. anti-aircraft.
--n. an anti-aircraft gun etc.
--adj. anti-aircraft.
--n. an anti-aircraft gun etc.
Etymology
formerly signallers' name for the letters AA
For further exploring for "ack-ack" in Webster Dictionary Online