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 ]
(adj) not easily perturbed or excited or upset; marked by extreme calm and composure, Syn. unflappable, Example:hitherto imperturbable, he now showed signs of alarm; an imperturbable self-possession; unflappable in a crisis