Source : Webster's Revised Unabridged Dictionary (1913)

Plain \Plain\, v. t. [imp. & p. p. {Plained}; p. pr. & vb. n.
   {Plaining}.] [Cf. {Plane}, v.]
   1. To plane or level; to make plain or even on the surface.
      [R.]

            We would rake Europe rather, plain the East.
                                                  --Wither.

   2. To make plain or manifest; to explain.

            What's dumb in show, I'll plain in speech. --Shak.