I was rather hoping some Yank or other would come into this thread to give their four cents' worth on the term in question. I visited the U.S. recently, and part of the itinerary was U.S.S. Nautilus, the first submarine to transit underneath the ice at the North Pole.
Short break for pub quiz question: How thick is the ice at the North Pole?
Now, where was I? Ah, yes, one of the flags / jacks / whatever the boat was proudly flying was what I, as a Brit, would describe as the Stars and Stripes without the Stars, but with a snake, and the motto, "Don't Tread on Me". I had seen it before, on the U.S.S. Constitution, so I asked about it.
I was informed, "It's our first Union Jack."
It sort of follows logic - Union, i.e. the U.S., Jack, i.e. flag (landlubber's term) on a pole. Unfortunately, I did not have the opportunity to enquire further, but can anyone give me the lowdown, please?