ack-ack

Etymology
From +, RAF WWI signalese for. (In modern signalese it would be called alpha-alpha.)

Noun

 * 1) Antiaircraft artillery.
 * The army fired upon us with ack-ack.
 * Ack-acks were set up in the square.
 * 1)  Antiaircraft fire.
 * The bomber flew through heavy ack-ack.
 * The bomber flew through heavy ack-ack.