Plain sailing

We all know what it means now - an easy, uncomplicated task of any description, and in this figurative sense, the expression dates from the late 18th/early 19th century, where plain means ordinary, easy or unobstructed, as in plain speaking. During the 1600s, however, the original, technical, nautical expression was ‘plane’ sailing. This was easy, uncomplicated sailing in calm seas where short distances, position, etc, could be worked out by means of simple trigonometry, as if sailing on a plane or flat surface and making no allowance for the curvature of the earth. By the time the expression became more widely used in its current figurative sense, the original nautical meaning had been forgotten. Therefore, the spelling plain was preferred through association with other similar phrases like plain spoken etc.