"The ack-ack will knock those planes out of the sky," one of the sailors said.
That was a score for the ack-ack gunners and the ground boys.
The Jerries were at it again and seemed to have slipped inside the balloons and the ring of ack-ack guns.
1939, representing A.A., the military abbreviation for anti-aircraft (see ack).
: ack-ack positions
Antiaircraft gun or guns; antiaircraft fire; aa, flak (WWI)