Jolly Roger is the traditional English name for the flags flown to identify a pirate ship about to attack during the early 18th century (i.e. the later part of the "Golden Age of Piracy").
The flag most commonly identified as the Jolly Roger today, the skull and crossbones symbol on a black flag, was used during the 1710s by a number of pirate captains including Black Sam Bellamy, Edward England, and John Taylor and it went on to become the most commonly used pirate flag during the 1720s.
Use of the term Jolly Roger in reference to pirate flags goes back to at least Charles Johnson's A General History of the Pyrates, published in Britain in 1724.
Johnson specifically cites two pirates as having named their flag "Jolly Roger": Bartholomew Roberts in June 1721 and Francis Spriggs in December 1723. While Spriggs and Roberts used the same name for their flags, their flag designs were quite different, suggesting that already "Jolly Roger" was a generic term for black pirate flags rather than a name for any single specific design. Neither Spriggs' nor Roberts' Jolly Roger consisted of a skull and crossbones.
Richard Hawkins, who was captured by pirates in 1724, reported that the pirates had a black flag bearing the figure of a skeleton stabbing a heart with a spear, which they named "Jolly Roger".
The origin of the name is unclear. Jolly Roger had been a generic term for a jovial, carefree man since at least the 17th century and the existing term seems to have been applied to the skeleton or grinning skull in these flags by the early 18th century. In 1703, a pirate named John Quelch was reported to have been flying the "Old Roger" off Brazil, "Old Roger" being a nickname for the devil.
It is sometimes claimed that the term derives from "Jolie Rouge" ("Pretty Red") in reference to a red flag used by French privateers. This hypothesis is considered a false etymology, as the phrase "Jolie Rouge" in reference to a pirate flag does not appear in any historical sources.