Late 17th century: from French, from earlier plant 'ground plan, plane surface', influenced in sense by Italian pianta 'plan of building'. Compare with plant.

• plain from Middle English:

  The source of both plain and plane is Latin planus ‘flat’. Mathematicians introduced the spelling plane in the early 17th century to distinguish the geometrical uses of plain from senses such as ‘ordinary’ and ‘simple’. The sort of plane used to make wood flat is from the same source. Plane meaning ‘an aircraft’ is unconnected, and is a shortening of aeroplane. Also unconnected is the plane tree (Late Middle English), which is not flat but ‘broad’, the meaning of its Greek source platus. The plan of a building (mid 17th century), which involves putting something three-dimensional on a flat surface, is, however, related. The earlier version of the expression as plain as a pikestaff, ‘very obvious’, was as plain as a packstaff, which gives a small clue as to its origins. A packstaff was a long stick which a peddler used to carry his pack of goods for sale, which would probably have been obvious from a distance as the peddler trudged along the road. By the end of the 16th century people had started to use the current version with pikestaff, and a hundred years later it had more or less taken over. A pikestaff was a walking stick with a pointed metal tip, which possibly replaced packstaff because it sounded similar and peddlers were becoming a less familiar sight. The phrase plain sailing, ‘smooth and easy progress’, probably represents a use of plane sailing, referring to the practice of determining a ship's position on the theoretical assumption that it is moving on a plane. Plain Jane first appears in 1912, in Carnival by Compton Mackenzie. There was probably no real Jane behind the phrase, just a fortunate rhyme.