Uncovered is a related term of plain.
As adjectives the difference between uncovered and plain
is that
uncovered is not covered or protected from the weather etc while
plain is .
As an adverb plain is
(colloquial) simply.
As a noun plain is
(rare|poetic) a lamentation or
plain can be an expanse of land with relatively low relief.
As a verb plain is
to lament, bewail or
plain can be (obsolete|transitive) to plane or level; to make plain or even on the surface.