Source : Webster's Revised Unabridged Dictionary (1913)

Smoothen \Smooth"en\, v. t.
   To make smooth. [Obs.]

Source : WordNet®

smoothen
     v 1: make smooth or smoother, as if by rubbing; "smooth the
          surface of the wood" [syn: {smooth}] [ant: {roughen}]
     2: (of surfaces) make shine; "shine the silver, please";
        "polish my shoes" [syn: {polish}, {smooth}, {shine}]
     3: become smooth