Definition of "impenetrable"

impenetrable

/ɪmˈpɛnətɹəbəl/

adjective
not comparable
1. Not penetrable.
  Quotations
  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.
  2012, John Branch, “Snow Fall : The Avalanche at Tunnel Creek”, in New York Time‎
2. (figuratively) Incomprehensible; fathomless; inscrutable.
3. Opaque; obscure; not translucent or transparent.
noun
plural impenetrables
1. A person not openly given to friendship. (clarification of this definition is needed)
  Quotations
  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.
  1837, L[etitia] E[lizabeth] L[andon], “Chapter XXVI. Lady Marchmont to Sir Jasper Meredith.”, in Ethel Churchill: Or, The Two Brides. […], volume I, London: Henry Colburn, […], page 236