impenetrable

adj

1. (not-comparable)Not penetrable.

   “The fortress is impenetrable, so it cannot be taken.”

   “The avalanche spread and stopped, locking everything it carried into an icy cocoon. It was now a jagged, virtually impenetrable pile of ice, longer than a football field and nearly as wide.”

2. (not-comparable)Opaque; obscure; not translucent or transparent.

   “When night falls, she cloaks the world in impenetrable darkness.”

3. (figuratively, not-comparable)Incomprehensible; fathomless; inscrutable.

   “Business jargon makes this document impenetrable—I can’t understand it.”

noun

1. A person not openly given to friendship.

   “I should lose the reputation that I am gradually acquiring among our impenetrables here, were I to confess the excitement which I felt at the idea of entering his house—the house of that great general under whose command you made your first charge.”