imperturbable (adj.)

late 14c., perturben, "disturb greatly, disturb mentally; cause disorder in," from Old French perturber "disturb, confuse" (14c.) and directly from Latin perturbare "to confuse, disorder, disturb," especially of states of the mind, from per "through" (from PIE root *per- (1) "forward," hence "through") + turbare "disturb, confuse," from turba "turmoil, crowd" (see turbid). Related: Perturbed; perturbing.

word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.