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]