/ ˌɪmpəˈtɜːbəbl; NAmE -pərˈtɜːrb-/ adjective
( formal ) not easily upset or worried by a difficult situation; calm
► im·per·turb·ability / ˌɪmpəˌtɜːbəˈbɪləti; NAmE -pərˌtɜːrb-/ noun [ U ]
► im·per·turb·ably / -əbli; NAmE / adverb
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WORD ORIGIN
late Middle English : from late Latin imperturbabilis , from in- not + perturbare (from per- completely + turbare disturb).