The area on old British warships below the upperdeck, to give a false perception of the actual amount of ammuniton onboard. This is supposedly how the term was derived.
Is this saying that they'd display a bunch of powder and shot on the gun deck during some sort of official visit, but have (nearly) empty magazines?
I took it to mean a below deck that actually had no cannons, but made to look like it did have cannons. A way to fool a distant enemy ship.