Wiktionary
n. (context nautical English) A short spar, at the bows of a ship, that serves as a flagpole to fly a flag (a ''jack'')
Wikipedia
A jack staff (also spelled as jackstaff) is a small vertical spar (pole) on the bow of a ship or smaller vessel on which a particular type of flag, known as a jack, is flown. The jack staff was introduced in the 18th century. The jack is typically flown from military vessels, including submarines, while at anchor or moored pierside, but not while underway. Civilian vessels such as private yachts have also been known to fly the jack of the nation of their homeport, also from a jackstaff, while moored or at anchor.
Usage examples of "jackstaff".
Kriegsmarine ensign on her jackstaff, vivid as she was swallowed by the mist like a ghost.
Wagner hauled down the Kriegsmarine ensign on the jackstaff above the wheelhouse.
The night was warm and no shelter was needed, although, as with most Saxon warships, it was possible to unship the light mast and drop the tip of it into the forked jackstaff at the prow.
Holding position effortlessly above the jackstaff was a Wandering Albatross.