The Collaborative International Dictionary of English v.0.48:

Insecure \In`se*cure"\, a.
   1. Not secure; not confident of safety or permanence;
      distrustful; suspicious; apprehensive of danger or loss.
      [1913 Webster]

            With sorrow and insecure apprehensions. --Jer.
                                                  Taylor.
      [1913 Webster]

   2. Not effectually guarded, protected, or sustained; unsafe;
      unstable; exposed to danger or loss. --Bp. Hurg.
      [1913 Webster]

            The trade with Egypt was exceedingly insecure and
            precarious.                           --Mickle.
      [1913 Webster]

WordNet (r) 3.0 (2006):

insecure
    adj 1: not firm or firmly fixed; likely to fail or give way;
           "the hinge is insecure" [ant: secure]
    2: lacking in security or safety; "his fortune was increasingly
       insecure"; "an insecure future" [syn: insecure, unsafe]
       [ant: secure]
    3: lacking self-confidence or assurance; "an insecure person
       lacking mental stability" [ant: secure, unafraid,
       untroubled]
    4: not safe from attack [syn: insecure, unsafe]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "insecure":
   afraid, anxious, apprehensive, dangerous, desultory, dickey,
   disconcerted, doubtful, dubious, exposed, fearful, flimsy,
   fluctuant, frail, hazardous, hesitant, in suspense, infirm,
   insubstantial, jumpy, nervous, open, perilous, precarious,
   provisional, questioning, rickety, risky, rocky, rootless, shaky,
   shifting, shifty, slippery, temporary, tentative, ticklish,
   tottery, treacherous, unassured, uncertain, uncomfortable,
   unconfident, undefended, undependable, unfaithworthy, unguarded,
   unhealthy, unnerved, unpredictable, unprotected, unreliable,
   unsafe, unselfassured, unselfconfident, unselfreliant, unsolid,
   unsound, unstable, unsteadfast, unsteady, unsubstantial, unsure,
   untrustworthy, vacillating, vulnerable, wavering, weak, wobbly,
   worried