means The quality of being plain — simple, unadorned, or unembellished, whether in appearance, manner, or speech.

from From 'plain' plus the noun-forming suffix '-ness.' 'Plain' came into English from Old French 'plain' (flat, even, clear), which traces back to Latin 'planus' (flat, level) — the same root that gives us 'plane' and 'plan.' The sense traveled from physically flat to figuratively unadorned: a level surface has nothing standing up on it, and plain things have nothing put on them. '-ness' is a homegrown Germanic suffix, old as English itself, that turns a quality into a thing you can name.