The Collaborative International Dictionary of English v.0.48:

Imperturbable \Im`per*turb"a*ble\, a. [L. imperturbabilis; pref.
   im- not + perturbare to disturb: cf. F. imperturbable. See
   Perture.]
   Incapable of being disturbed or disconcerted; as,
   imperturbable gravity.
   [1913 Webster]

WordNet (r) 3.0 (2006):

imperturbable
    adj 1: not easily perturbed or excited or upset; marked by
           extreme calm and composure; "hitherto imperturbable, he
           now showed signs of alarm"; "an imperturbable self-
           possession"; "unflappable in a crisis" [syn:
           imperturbable, unflappable]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "imperturbable":
   balanced, bovine, collected, complacent, composed, cool,
   dispassionate, dull, even-tempered, fast, fiducial, firm,
   firm as Gibraltar, impassive, in equilibrium, inexcitable,
   inirritable, nonchalant, patient, predictable, reliable, secure,
   self-satisfied, smug, solid, sound, stable, steadfast, steady,
   stoic, stolid, substantial, unaffected, undisturbable, unflappable,
   unflinching, unirritable, unmoved, unnervous, unpassionate,
   unshakable, untouched, unwavering, well-balanced, without nerves