Lewis Short
(adjective) : imperturbātus (inp-), a, um, 2. in-perturbatus
* Undisturbed, unruffled, calm (very rare): imperturbato ore,Ov. Ib. 562: publicis occupationibus quies,Sen. Ep. 73 med.—Of persons: audio imperturbatus, interritus,Plin. Ep. 9, 13, 8: qui constans est, inperturbatus est,Sen. Ep. 85, 2; cf. ib. § 3.
Charlton T. Lewis, Charles Short, A Latin Dictionary