13 letters · 21 points in Scrabble
adjective not easily perturbed or excited or upset; marked by extreme calm and composure
originPIE 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)).…
Etymology from Wiktionary, CC BY-SA 3.0