The Collaborative International Dictionary of English v.0.48:

Unperturbed \Unperturbed\
   See perturbed.

WordNet (r) 3.0 (2006):

unperturbed
    adj 1: free from emotional agitation or nervous tension; "the
           waiters were unflurried and good natured"; "with
           contented mind and unruffled spirit"- Anthony Trollope
           [syn: unflurried, unflustered, unperturbed,
           unruffled]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "unperturbed":
   at rest, calm, cloistered, cool, dwindling, ebbing, even-tenored,
   halcyon, hushed, impassive, isolated, moldering, pacific,
   peaceable, peaceful, placid, quiescent, quiet, reposeful, reposing,
   restful, resting, secluded, sequestered, sequestrated, sheltered,
   smooth, still, still as death, stillish, stilly, stoic, stolid,
   subsiding, tranquil, unagitated, unbothered, undisturbed,
   unexcited, unflustered, unimpassioned, unmoved, unplagued,
   unruffled, unstirred, unstirring, untormented, untroubled, unvexed,
   unworried, waning