imperturbable

PIE word *né From Late Middle English imperturbable (“undisturbed; impossible to disturb”), borrowed from Late Latin imperturbābilis, from Latin im- (variant of in- (prefix meaning ‘not’)) + Late Latin perturbabilis (“perturbable”) (from Latin perturbō (“to confuse; to alarm, disturb, trouble, perturb”) + -bilis (suffix forming adjectives denoting a capacity or worth of being acted upon)). Perturbō is derived from per- (intensifying prefix) + turbō (“to agitate, disturb, unsettle, perturb; to upset”) (from turba (“disorder, disturbance, turmoil”) (possibly from Ancient Greek τῠ́ρβη (tŭ́rbē, “confusion, disorder, tumult”), either from Pre-Greek, or Proto-Indo-European *(s)twerH- (“to agitate, stir up; to urge on, propel”)) + -ō (suffix forming infinitives of regular first-conjugation verbs)). By surface analysis, im- (prefix meaning ‘not’) + perturbable.

来源：wiktionary