im·per·turb·a·ble /ˌimpərˈtərbəb(ə)l/ adjective unable to be upset or excited; calm. "an imperturbable tranquility" synonyms: self-possessed, composed, collected, calm, cool, calm, and collected, as cool as a cucumber, coolheaded, self-controlled, poised, tranquil, serene, relaxed, easygoing, unexcitable, even-tempered, placid, sedate, phlegmatic; More