late 13c. (c.1200 as a surname), from Old North French planke, 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 PIE *plak- (1) "to be flat" (see placenta). Technically, timber sawed to measure 2 to 6 inches thick, 9 inches or more wide, and 8 feet or more long. Political sense of "item of a party platform" is U.S. coinage from 1848. To walk the plank, supposedly a pirate punishment, is first attested 1789 and most early references are to slave-traders disposing of excess human cargo in crossing the ocean.
To behave quietly and peacefully; be modest: I told him he was acting like an ass, and he walks a lot softer now (1960s+ Black)
To do the sex act with or to; screw: had witless good fun with his children while his wife was out getting planked
[1970s+; origin unknown]