The Collaborative International Dictionary of English v.0.48:

Assuasive \As*sua"sive\, a. [From assuage, as if this were fr. a
   supposed L. assuadere to persuade to; or from E. pref. ad +
   -suasive as in persuasive.]
   Mitigating; tranquilizing; soothing. [R.]
   [1913 Webster]

         Music her soft assuasive voice applies.  --Pope.
   [1913 Webster]

WordNet (r) 3.0 (2006):

assuasive
    adj 1: freeing from fear and anxiety [syn: assuasive,
           soothing]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "assuasive":
   alleviating, alleviative, altering, analgesic, anesthetic, anodyne,
   balmy, balsamic, benumbing, bounding, calmant, calmative,
   cathartic, cleansing, deadening, demulcent, dulling, easing,
   emollient, extenuating, extenuatory, lenitive, limitative,
   limiting, mitigating, mitigative, mitigatory, modificatory,
   modifying, modulatory, numbing, pain-killing, palliative,
   purgative, qualificative, qualificatory, qualifying, relieving,
   remedial, restricting, restrictive, sedative, softening, soothing,
   subduing