1 Crows Nest: Sail off the port bow! She's flying the Jolly Roger!
2 Ponsonby: Now, Captain Long Tom Short, let's see how you do against the firepower of this fully armed and operational battle station!
3 GM: {glare}
4 Ponsonby: Erm... against the cannon of a fully gunned and crewed British galleon!

A little movie you may have heard of.

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2017-04-01 Rerun commentary: Jolly Roger is the name given to any of several flag designs flown by pirates during the early 18th century. Today, most people commonly think of it as the white skull and crossbones on a black field, but several other designs and variants were also used by pirates, and all qualify as being Jolly Rogers. See the historical designs section of the Wikipedia page linked above.

Use of flags resembling Jolly Rogers did not die out with the end of the Age of Piracy. In particular, a tradition arose in World War I of naval submarines flying a skull and crossbones flag under particular circumstances. This tradition arose because of comments by Admiral Sir Arthur Wilson during his appointment as First Sea Lord (i.e. head of the British Navy) from 1910-1911, that submarines were "underhanded, unfair, and damned un-English". He mounted a campaign to have all captured crew of enemy submarines hanged as pirates.

Wilson did not leave a well-regarded legacy in the Navy, and of course Britain went on to field several submarines during World War I. In September 1914, the submarine HMS E9 successfully torpedoed and sank the German light cruiser SMS Hela. Remembering Wilson's pronouncement about submariners being pirates, the captain, Lieutenant-Commander Max Horton, decided to make an ironic statement by instructing his crew to make a Jolly Roger, which they flew as the submarine returned to port. The submarine flew additional Jolly Rogers after each successful patrol, until eventually the commander had a single large Jolly Roger made, with space for badges to be sewn on after sinking additional ships.

This habit caught on with other submarines in the Navy during World War I, and the practice revived in the British Navy in World War II. British submarines have since flown the Jolly Roger after other engagements, including the Falklands War and the Gulf War, and will no doubt continue in the future, given how much the British love traditions.^[1]

[1] A few web sites state that the Jolly Roger has been adopted as an official emblem of the Royal Navy Submarine Service, including Wikipedia, which cites one of these sites. However, I could find no confirmation of this on any RNSS or British Government site, so I am treating it with some scepticism.