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®

impassively
     adv : in an impassive manner; "he submitted impassively to his
           arrest"