Definition of "impenetrability"

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.

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,”

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

1904, Joseph Conrad, Nostromo‎, Part 2, Chapter Five

He watched intently the place where she must appear; it would give him pointers about the impenetrability of mist to the eye.

1924, Ford Madox Ford, Some Do Not ..., Part I, Chapter 7, in Parade's End, New York: Knopf, 1950, p. 137

If everybody came to agree that stories should be told this clearly, the professors of literature would be out of a job, and the writers of obscure, encoded fiction would be, not honored, but pitied for their impenetrability.

1991, Orson Scott Card, Ender's Game (introduction to revised edition), Tor 1991

As he passed gleaming wooden door after gleaming wooden door, each bearing a small plaque with the owner’s name and occupation upon it, the might of the Ministry, its complexity, its impenetrability, seemed to force itself upon him so that the plan he had been carefully concocting with Ron and Hermione over the past four weeks seemed laughably childish.

2007, J. K. Rowling, chapter 13, in Harry Potter and the Deathly Hallows, New York: Scholastic, page 247