WordNet (r) 3.0 (2006):

unflappable
    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:

59 Moby Thesaurus words for "unflappable":
   balanced, bovine, collected, composed, constant, cool,
   dispassionate, dull, easy, even-tempered, fast, fiducial, firm,
   firm as Gibraltar, fixed, flinty, immovable, impassive,
   imperturbable, in equilibrium, inexcitable, inflexible,
   inirritable, loyal, nonchalant, patient, predictable, relaxed,
   reliable, secure, set, settled, solid, sot, sound, stable, staunch,
   steadfast, steady, steely, stoic, stolid, substantial, true,
   unbending, undeflectable, undisturbable, unflinching, unirritable,
   unnervous, unpassionate, unruffled, unshakable, unshaken,
   unswerving, unwavering, unyielding, well-balanced,
   without nerves