late 13c. (c. 1200 as a surname), "thick board used in construction," from Old North French planke, a variant of Old French planche "plank, slab, little wooden bridge" (12c.), from Late Latin planca "broad slab, board," probably from Latin plancus "flat, flat-footed," from a nasalized variant of PIE root *plak- (1) "to be flat." Planche itself was also used in Middle English.
Technically, timber sawed to measure 2 to 6 inches thick, 9 inches or more wide, and 8 feet or more long. The political sense of "article or paragraph formulating a distinct principle in a party platform" is U.S. coinage from 1848, based on the double sense of platform. To be made to walk the plank, "be forced to walk along a plank laid across the bulwarks of a ship until one reaches the end and falls into the sea," popularly supposed to have been a pirate form of execution, is attested from 1789, and most early references are to slave-ships disposing of excess human cargo in crossing the ocean.