DuckCorp Dico

Found one definition

1.                 From English - German Ding/FreeDict dictionary ver. 1.9-fd1:
                   
   
   
                   
                     
   ack-ack /ˈak ˈak/ (AA /ˈɑː/)
   Flugabwehr…, Fliegerabwehr… [Ös.]  [Schw.] , Fla…, Luftabwehr… [mil.]
         "ack-ack fire"  - Flugabwehrfeuer, Fla-Feuer
      Synonym: anti-aircraft