[1] : The Collaborative International Dictionary of English v.0.48

smoothen \smooth"en\, v. t.
   to make smooth. obs.
   1913 webster

[2] : WordNet (r) 2.0

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
see also:
smooth roughen polish shine 

Smoothen >> S Words
Smoothen, definition of term: Smoothen
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