reticent

from WordNet (r) 3.0 (2006)

reticent
    adj 1: temperamentally disinclined to talk [syn: {reticent},
           {untalkative}]
    2: cool and formal in manner [syn: {restrained}, {reticent},
       {unemotional}]
    3: reluctant to draw attention to yourself [syn: {reticent},
       {self-effacing}, {retiring}]

from The Collaborative International Dictionary of English v.0.48

Reticent \Ret"i*cent\, a. [L. reticens, p. pr. of reticere to
   keep silence; re- + tacere to be silent. See {Tacit}.]
   Inclined to keep silent; reserved; uncommunicative.
   [1913 Webster]

from Moby Thesaurus II by Grady Ward, 1.0

55 Moby Thesaurus words for "reticent":
      Olympian, aloof, backward, bashful, blank, chilled, chilly, close,
      close-lipped, close-tongued, closemouthed, cold, constrained, cool,
      detached, discreet, distant, expressionless, forbidding, frigid,
      frosty, guarded, icy, impassive, impersonal, inaccessible,
      introverted, modest, offish, quiet, remote, removed, repressed,
      reserved, restrained, retiring, shrinking, shy, silent, standoff,
      standoffish, subdued, suppressed, taciturn, tight-lipped, timid,
      unaffable, unapproachable, uncommunicative, uncongenial,
      undemonstrative, unexpansive, ungenial, unresponsive, withdrawn