Other forms: planks
A plank is a long flat piece of sawn timber. Thicker than a board, and long enough to walk along.
Building and boat-building use it precisely, and pirates supposedly made people walk the plank, which is where the phrase comes from. Politics borrowed it for a policy, so a plank of the manifesto is one of the things it stands on. Exercise took the word for a position held face down on the elbows. British informal speech uses it as a mild insult, since calling somebody a plank means they are being stupid.
Ancient Greek
φάλαγξ
Latin
phalanga
Vulgar Latin
planca
fro-nor
planque
Old French
planke
Middle English
plank
English, today
plank
From Middle English plank, planke, borrowed from Old French planke, fro-nor planque (compare French planche, from Old French planche), from Vulgar Latin planca, from palanca, from Latin phalanga. The Latin term derives from the Ancient Greek φάλαγξ, so it is thus a a doublet of phalange and phalanx. Compare also the doublets a doublet of planch and planche and plancha, borrowed later from Middle French, Modern French, and Spanish, respectively.
Etymology from Wiktionary, CC BY-SA