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Webster 1913 Edition

adv.

In a plain manner; clearly.

adv.

With a level surface. [Little used.]

Without cunning or disguise.

Without ornament or artificial embellishment; as, to be plainly clad.

Frankly; honestly; sincerely; as, deal plainly with me.

In earnest; fairly.

In a manner to be easily seen or comprehended.

Thou shalt write on the stones all the words of this law very plainly. Deut.27.

Evidently; clearly; not obscurely. The doctrines of grace are plainly taught in the Scriptures.

plainly ‎(comparative plainlier or more plainly, superlative plainliest or most plainly)

In a plain manner; simply; basically.
She decorated the room plainly but neatly.‎
Obviously; clearly.
- 1898, Winston Churchill, chapter 1, in The Celebrity:
  The stories did not seem to me to touch life. They were plainly intended to have a bracing moral effect, and perhaps had this result for the people at whom they were aimed. They left me with the impression of a well-delivered stereopticon lecture, with characters about as life-like as the shadows on the screen, and whisking on and off, at the mercy of the operator.
You will see that ours is plainly the better method.‎

In a plain manner

Obviously; clearly