I.that cannot be taken by assault, impregnable, inexpugnable (class.)
II. Trop., unconquerable, in assured safety: “homo,” Cic. Tusc. 5, 14, 41: “terra,” impenetrable, Plin. 33, 4, 21, § 72: “gramen,” that cannot be rooted out, Ov. M. 5, 486: “pectus Amori,” id. ib. 11, 767: “finitio verborum,” Quint. 7, 3, 18: “necessitas dormiendi,” Cels. 3, 20.