The Collaborative International Dictionary of English v.0.48:

Lessener \Less"en*er\ (-[~e]r), n.
   One who, or that which, lessens.
   [1913 Webster]

         His wife . . . is the lessener of his pain, and the
         augmenter of his pleasure.               --J. Rogers
                                                  (1839).
   [1913 Webster]