Short for acknowledged. When a person says something that doesn't require a response, but the person wants to let them know they recieved the message. Similar to okay, but when using okay just won't do.
Sanford: I had cheerios for breakfast today.
Stephen: I don't care
Sanford: Ack
By Livvy
Ack-ack
Noun: Stemming from the slang for "anti-aircraft" - A person who shoots another person's idea, thought, or feeling down.
Person 1: "How about Joe's Bar tonight?"
Person 2: "How about no?"
Person 1: "You, sir, are an Ack-ack..."