Walking the plank was a form of murder or torture thought to have been practiced by pirates, mutineers and other rogue seafarers. The victim was forced to walk off the end of a wooden plank or beam extended over the side of a ship, falling into the water to drown, sometimes with bound hands or weighed down, often into the vicinity of sharks (which would often follow ships).
The earliest known use of the phrase is the latter half of the 18th century. Some writers in the 20th century speculated that walking the plank may be a myth created by cinema; however, the phrase "walking the plank" is recorded in English writer Francis Grose's "Dictionary of the Vulgar Tongue", which was published in 1788 (first published in 1785).