What is ack-ack?

This is slang for flak, sometimes called AA fire, othertimes known as tirple A (or AAA anti-aircraft-artillery). AA stands for Anti-Air fire. The German name for AA is Fliegerabwehr. The name flak came from the German name Flugabwehrkanone. Which loosely translates as air defense cannon. Others say the name came from the sound of flak making an ack ack sound. 'Ack, ack' does not come from German, but from the alphabet used by British signallers in the First World War. The alphabet involved saying 'ack' for 'a', 'beer' for 'b' and so on...hence AA when transmitted by signallers would be transmitted as 'Ack, ack'.

Both. German 88 also called in basic German Acht-acht, eight-eight and also the British use ack-ack. Although the latter is more likely what is being referred during WWI and WWII, seeing that both refer to AA, with the former being more specific, I'd say, this information just gives more insight of use.