Walking the plank was a method of execution practiced on special occasion by pirates, mutineers, and other rogue seafarers. For the amusement of the perpetrators (and the psychological torture of the victims), captives were bound so they could not swim or tread water and forced to walk off a wooden plank or beam extended over the side of a ship.
Although forcing captives to walk the plank is a motif of pirates in popular culture in the 19th through 21st centuries, few instances are documented, so the frequency of the practice is uncertain.
Walking the plank. A mode of destroying devoted persons or officers in a mutiny on ship-board, by blind-folding them, and obliging them to walk on a plank laid over the ship's side; by this means, as the mutineers suppose, avoiding the penalty of murder.
In 1769, mutineer George Wood confessed to his chaplain at London's Newgate Prison that he and his fellow mutineers had sent their officers to walk the plank. Author Douglas Botting, in describing the account, characterized it as an "alleged confession" and an "obscure account... which may or may not be true, and in any case had nothing to do with pirates".
A Mr. Claxton, surgeons-mate aboard the Garland in 1788, testified to a committee at the House of Commons about the use of the plank by slavers:
The food, notwithstanding the mortality, was so little, that if ten more days at sea, they should, as the captain and others said, have made the slaves walk the plank, that is, throw themselves overboard, or have eaten those slaves that died.
In Charles Gayarré's 1872 novel Fernando de Lemos: Truth and Fiction, the pirate Dominique Youx confessed to capturing the schooner Patriot, killing its crew and making its passenger, Theodosia Burr Alston (June 21, 1783 - approximately January 2 or 3, 1813) walk the plank. "She stepped on it and descended into the sea with graceful composure, as if she had been alighting from a carriage," Gayarré wrote in Youx's voice. "She sank, and rising again, she, with an indescribable smile of angelic sweetness, waved her hand to me as if she meant to say: 'Farewell, and thanks again'; and then sank forever." Because Gayarré mixed fact with fiction, it unknown whether Youx's confession was real or not. 
The Times of London reported on February 14, 1829 that the packet Redpole (Bullock, master) was captured by the pirate schooner President and sunk. The commander was shot and the crew were made to walk the plank.
In 1829, pirates intercepted the Dutch brig Vhan Fredericka in the Leeward Passage between the Virgin Islands, and murdered most of the crew by making them walk the plank with cannonballs tied to their feet.
It was said that forcing loyal seamen to walk the plank was supposed, by the perpetrators, to "avoid the penalty for murder", but this legal argument would hardly have worked. Not only would most legal authorities not have hesitated to prosecute any person who forced another to his death, but piracy and mutiny were also capital crimes. Given the occasions on which it was known to have been employed, it appears more likely to have been an elaborate and unusual form of sadistic entertainment rather than a regular method of murdering unwanted captives.[original research?]
Despite the likely rarity of the practice in actual history, walking the plank entered popular myth and folklore via depictions in popular literature.
Captain Charles Johnson, in his 1724 book A General History of the Pyrates, described a similar practice (using a ladder rather than a plank) in the Mediterranean of classical antiquity - Roman captives were offered the ladder and given their freedom, provided they were willing to swim for it.
The title page of Charles Ellms's sensationalist 1837 work The Pirates Own Book, apparently drawing on Charles Johnson's description, contains an illustration titled "A Piratical Scene - 'Walking the Death Plank'".
Robert Louis Stevenson's 1884 classic Treasure Island contains at least three mentions of walking the plank, including at the beginning where Billy Bones tells bone-chilling stories of the practice to Jim Hawkins. (Treasure Island also popularized other now-common pirate motifs such as parrots, peglegs, and buried treasure.)
The notion of 'walking the plank' is a myth...