From BiblePortal Wikipedia

King James Dictionary [1]

Pla'Inly, adv. With a level surface. Little used.

1. Without cunning or disguise. 2. Without ornament or artificial embellishment as, to be plainly clad. 3. Frankly honestly sincerely as, deal plainly with me. 4. In earnest fairly. 5. In a manner to be easily seen or comprehended.

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

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

Webster's Dictionary [2]

(adv.) In a plain manner; clearly.