Source : Webster's Revised Unabridged Dictionary (1913)

Impassive \Im*pas"sive\, a.
   Not susceptible of pain or suffering; apathetic; impassible;
   unmoved.

         Impassive as the marble in the quarry.   --De Quincey.

         On the impassive ice the lightings play. --Pope.
   -- {Im*pas"sive*ly}, adv. -- {Im*pas"sive*ness}, n.

Source : WordNet®

impassiveness
     n : apathy demonstrated by an absence of emotional reactions
         [syn: {emotionlessness}, {impassivity}, {phlegm}, {indifference},
          {stolidity}, {unemotionality}]