Emily Dickinson Lexicon

PLANK, n. [Fr. planche; Arm. plancquenn, plur. plench; W. planc; D. plank; G. and Dan. planke; Sw. planka; Russ. placha, a board or plank. Probably n is casual and the word belongs to Class Lg.]

A broad piece of sawed timber, differing from a board only in being thicker. In America, broad pieces of sawed timber which are not more than an inch or an inch and a quarter thick, are called boards; like pieces from an inch and a half to three or four inches thick, are called planks. Sometimes pieces more than four inches thick are called planks.