The Collaborative International Dictionary of English v.0.48:

Impassivity \Im`pas*siv"i*ty\, n.
   The quality of being insusceptible of feeling, pain, or
   suffering; impassiveness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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