Lewis Short
(adjective) : impĕnĕtrābĭlis (inp-), e, 2. inpenetrabilis
* That cannot be penetrated, impenetrable (perh. not ante-Aug.).
* Lit.: silex ferro,Liv. 36, 25: superior pars corporis crocodili (with dura),Sen. Q. N. 4, 2 med.; cf.: tergus hippopotami ad scuta,Plin. 8, 25, 39, § 95: congeries imbribus,id. 8, 36, 54, § 127: cruppellarii accipiendis ictibus,Tac. A. 3, 43: quae impenetrabilia quaeque pervia,id. ib. 12, 35.
* Trop., that cannot be overcome, unconquerable, unyielding: patet impenetrabilis ille Luctibus,Sil. 6, 413: impenetrabilis blanditiis,Sen. Q. N. 4 praef.: mens irae,Sil. 7, 561: pudicitia Agrippinae,Tac. A. 4, 12.
Charlton T. Lewis, Charles Short, A Latin Dictionary