The Collaborative International Dictionary of English v.0.48:

Coarsen \Coars"en\ (k[=o]rs"'n), v. t.
   To make coarse or vulgar; as, to coarsen one's character.
   [R.] --Graham.
   [1913 Webster]

WordNet (r) 3.0 (2006):

coarsen
    v 1: make or become coarse or coarser; "coarsen the surface";
         "Their minds coarsened"
    2: make less subtle or refined; "coarsen one's ideals"

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "coarsen":
   adulterate, alloy, boss, brutify, canker, carnalize, cheapen,
   confound, contaminate, corrupt, debase, debauch, defile, deflower,
   degenerate, degrade, denature, deprave, desecrate, despoil,
   devalue, distort, gnarl, grain, granulate, horripilate, infect,
   knob, misuse, pervert, pimple, poison, pollute, popularize,
   prostitute, ravage, ravish, rough, rough up, roughen, smooth, stud,
   taint, tooth, twist, ulcerate, violate, vitiate, vulgarize, warp