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imperturbable (adj.)

c. 1500, from French imperturbable (15c.) and directly from Late Latin imperturbabilis "that cannot be disturbed" (Augustine), from assimilated form of in- "not, opposite of" (see in- (1)) + *perturbabilis, from Latin perturbare "to confuse, disturb" (see perturb). Related: Imperturbably (1785); imperturbability (1831; earlier as a dictionary word); imperturbation.

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Definitions of imperturbable from WordNet

imperturbable (adj.)
not easily perturbed or excited or upset; marked by extreme calm and composure;
an imperturbable self-possession
hitherto imperturbable, he now showed signs of alarm
Synonyms: unflappable
From wordnet.princeton.edu