"metal disk out of which a coin is made," 1610s, from French planchette, literally "a small board," a diminutive of Old French planche (12c.; the source of plank), from Late Latin planca "board, slab, plank," which is probably from Latin plancus "flat, flat-footed," from a nasalized variant of the PIE root *plak- (1) "to be flat."
The small, heart-shaped planchette on its three legs, used in automatism and on Ouija boards, is a re-borrowing of the French word, by 1860.
If the tips of the fingers of one person, or of two, are placed lightly upon it, the board will often, after a time, move without conscious effort on the part of the operator, and the pencil-point will, it is said, trace lines, words, and even sentences. It was invented about 1855, and was for a time an object of not a little superstition. [Century Dictionary]