Английский Этимологический словарь - plan (n.)

- 1678, from Fr. plan "ground plan, map," lit. "plane surface," from L. planum "level or flat surface," from planus "level, flat." Meaning "scheme of action, design," as well as that of "drawing of a design" are both first recorded 1706, from either the notion of drawing on a "flat surface," or possibly from Fr. planter "to plant," from It. planta "ground plan." The verb is first recorded 1728.