The Collaborative International Dictionary of English v.0.48:

ack-ack \ack-ack\ n. [from AA, meaning antiaircraft pronounced
   ack-ack by British signalmen.] (Mil.)
   Artillery designed to shoot upward at airplanes; antiaircraft
   artillery.

   Syn: antiaircraft, antiaircraft gun, flak, pompom, pom-pom,
        ack-ack gun
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

ack-ack
    n 1: artillery designed to shoot upward at airplanes [syn:
         antiaircraft, antiaircraft gun, flak, flack, pom-
         pom, ack-ack, ack-ack gun]