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..."
-Anti air flak, term used in World War I.
Ack-ack downed more planes than dogfights during the First World War.