-ˈtərbəbəl adjective
Etymology: Middle English, from Late Latin imperturbabilis, from Latin in- in- (I) + perturbare to perturb + -abilis -able — more at perturb
: marked by extreme calm, impassivity, assurance, and steadiness : unlikely to be disconcerted, agitated, or alarmed
hitherto imperturbable , he now showed signs of alarm
an imperturbable self-possession — Albert Dasnoy
Synonyms: see cool