Definition of ack-ack noun from the Oxford Advanced Learner's Dictionary

 

ack-ack

 noun
noun
BrE BrE//ˌæk ˈæk//
 
; NAmE NAmE//ˌæk ˈæk//
 
[uncountable](informal)
 
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the repeated firing of guns at aircraft Word Originsignallers' name for the letters AA; ack for A was replaced in military use by able in 1942.