In the 16th century, according to The Oxford Companion to Ships and The Sea, nautical charts were "drawn on the assumption that the earth was flat, although all experienced seamen knew this was not the case."
Practical Navigation - Plane Navigation, mid 19th century (x)
These charts were called planes, a word derived from the Latin planum (a flat surface). Plane (also spelled plain until the 18th century) sailing charts did not employ the priciples of parallels of latitude and meridians of longitude at right angles, required few calculations, and were, therefore, comparatively easy to use. Which means, if the course and distance are known, the latitude difference ΔφAB between A and B can be determined, as can the deviation, i.e. the distance to the east or west. The difference in longitude ΔλAB is not known and must be calculated with meridian parts, as in Mercator sailing.
Here you can read it in detail
On land, "plain sailing" became synonymous with easy progress on a straight, unobstructed course, which can also be attributed to easy calculation.
