im·per·turb·able \-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