impenetrability

noun

1. The characteristic of being impenetrable; invulnerability.

   “1651, uncredited translator, Naturall Philosophie Reformed by Divine Light, or, A Synopsis of Physicks by J. A. Comenius, London: Thomas Pierrepont, Chapter 4, “Of the tangible quality,” Humidity (or humour) is the liquidnesse of the parts of the body, and aptnesse to be penetrated by one another; siccity on the contrary is a consistency, and an impenetrability of the parts of the body.”

   “What does Don Carlos Gould think of that? But, of course, with his English impenetrability, nobody can tell what he thinks.”