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]

WordNet (r) 3.0 (2006):

impassive
    adj 1: having or revealing little emotion or sensibility; not
           easily aroused or excited; "her impassive remoteness";
           "he remained impassive, showing neither interest in nor
           concern for our plight"- Nordhoff & Hall; "a silent
           stolid creature who took it all as a matter of
           course"-Virginia Woolf; "her face showed nothing but
           stolid indifference" [syn: impassive, stolid]
    2: deliberately impassive in manner; "deadpan humor"; "his face
       remained expressionless as the verdict was read" [syn:
       deadpan, expressionless, impassive, poker-faced,
       unexpressive]

Moby Thesaurus II by Grady Ward, 1.0:

183 Moby Thesaurus words for "impassive":
   Olympian, affectless, aloof, anesthetized, apathetic, arctic,
   at rest, autistic, backward, bashful, blank, blunt, bored, bovine,
   callous, calm, careless, catatonic, chill, chilled, chilly,
   cloistered, cold, cold as charity, cold-blooded, coldhearted,
   collected, composed, constrained, contained, controlled, cool,
   deadpan, detached, discreet, disinterested, dispassionate, distant,
   drugged, dry, dull, dwindling, ebbing, emotionally dead,
   emotionless, empty, even-tempered, even-tenored, expressionless,
   fishy, forbidding, frigid, frosted, frosty, frozen, glassy,
   guarded, halcyon, hardened, heartless, heedless, hushed, icy,
   immovable, impassible, impersonal, imperturbable, inaccessible,
   incurious, indifferent, indurated, inexcitable, inexpressive,
   inirritable, insensible, insensitive, insentient, insouciant,
   insusceptible, introverted, isolated, lackadaisical, listless,
   matter-of-fact, mindless, modest, moldering, nonchalant,
   nonemotional, objective, obtuse, offish, out of touch, pacific,
   passionless, patient, peaceable, peaceful, phlegmatic, placid,
   poker-faced, quiescent, quiet, regardless, remote, removed,
   reposeful, reposing, repressed, reserved, restful, resting,
   restrained, reticent, retiring, secluded, self-absorbed,
   sequestered, sequestrated, serene, sheltered, shrinking, smooth,
   soulless, spiritless, standoff, standoffish, steady, still,
   still as death, stillish, stilly, stoic, stoical, stolid, stony,
   subdued, subsiding, suppressed, taciturn, tranquil, unaffable,
   unaffectionate, unagitated, unapproachable, uncaring, unconcerned,
   uncongenial, undemonstrative, undisturbable, undisturbed,
   unemotional, unexcitable, unexpansive, unexpressive, unfeeling,
   unflappable, ungenial, unimpassioned, unimpressible,
   unimpressionable, uninquiring, uninterested, uninvolved,
   unirritable, unloving, unmindful, unmoved, unnervous, unpassionate,
   unperturbed, unresponding, unresponsive, unruffled, unstirring,
   unsusceptible, unsympathetic, untouchable, untroubled, vacant,
   waning, withdrawn, wooden