plain as a pikestaff very plain. The phrase was originally (in the mid 16th century) plain as a packstaff, a packstaff being the staff on which a pedlar supported his wares while resting.
plain living and high thinking denoting a frugal and philosophic lifestyle; the original allusion is to Wordsworth's lines ‘Plain living and high thinking are no more: The homely beauty of the good old cause Is gone.’
Plain People the Amish, the Mennonites, and the Dunkers, three strict Christian sects emphasizing a simple way of life.
plain sailing used to describe a process or activity which goes well and is easy and uncomplicated. The phrase, which is mid 18th century, is probably a popular use of plane sailing, denoting the practice of determining a ship's position on the theory that it is moving on a plane.