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]

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]

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   undemonstrative, unexpansive, ungenial, unresponsive, withdrawn