The Collaborative International Dictionary of English v.0.48:

Impassive \Im*pas"sive\, a.
   Not susceptible of pain or suffering; apathetic; impassible;
   unmoved.
   [1913 Webster]

         Impassive as the marble in the quarry.   --De Quincey.
   [1913 Webster]

         On the impassive ice the lightings play. --Pope.
   -- Im*pas"sive*ly, adv. -- Im*pas"sive*ness, n.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

impassiveness \im*pas"sive*ness\ n.
   an absence of emotion.

   Syn: apathy, impassivity.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

impassiveness
    n 1: apathy demonstrated by an absence of emotional reactions
         [syn: emotionlessness, impassivity, impassiveness,
         phlegm, indifference, stolidity, unemotionality]