The Collaborative International Dictionary of English v.0.48:

Undemonstrative \Undemonstrative\
   See demonstrative.

WordNet (r) 3.0 (2006):

undemonstrative
    adj 1: not given to open expression of emotion [ant:
           demonstrative]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "undemonstrative":
   Olympian, actual, aloof, aseptic, backward, bashful, blank, chill,
   chilled, chilly, cold, constrained, cool, detached, discreet,
   distant, emotionless, expressionless, forbidding, frigid, frosty,
   glacial, guarded, icy, impassive, impersonal, inaccessible,
   incontestable, incontrovertible, indifferent, indisputable,
   indubitable, introverted, modest, offish, real, remote, removed,
   repressed, reserved, restrained, reticent, retiring, shrinking,
   standoff, standoffish, subdued, suppressed, true, unaffable,
   unapproachable, uncongenial, uncontestable, undeniable,
   unemotional, unequivocal, unexpansive, ungenial, uninterested,
   unquestionable, veridical, withdrawn