means Simple, ordinary, or unadorned — or, as a noun, a broad expanse of flat, level land.

from From Old French 'plain,' descended from Latin 'planus,' meaning flat, level, or even. The same root spreads out into 'plane,' 'plain,' and 'esplanade' — all sharing that sense of an unbroken, level surface. The figurative leap from 'flat ground' to 'plain speech' or a 'plain face' is old: what is level is also undecorated, straightforward, without hidden bumps.