From Latin 'plānus' (flat, level), via Old French — flatness became smoothness, then simplicity, then clarity.
Not decorated or elaborate; simple and unadorned; clear and easy to understand.
From Old French 'plain' meaning 'flat, smooth, simple,' from Latin 'plānus' meaning 'flat, level, even.' The sense progression: flat surface → smooth → undecorated → simple → obvious. Key roots: plānus (Latin: "flat, level").