impervĭus

I. that cannot be passed through, impassable, impervious (poet. and in post-Aug. prose; cf.: inaccessus, invius): amnis, Ov. M. 9, 106: iter, Quint. 12, 11, 11; cf. itinera (with interrupta), Tac. A. 3, 31: tellus, Val. Fl. 2, 643.— Transf.: lapis ignibus, Tac. A. 15, 43.