The Collaborative International Dictionary of English v.0.48:

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.]
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            We would rake Europe rather, plain the East.
                                                  --Wither.
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   2. To make plain or manifest; to explain.
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            What's dumb in show, I'll plain in speech. --Shak.
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