imperturbable (adj.) Look up imperturbable at Dictionary.com
c.1500, from Middle French imperturbable 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; imperturbability.