From Middle English inpenetrabel, inpenetrabyle, from Middle French impenetrable or directly from its etymon, Latin impenetrābilis.

• IPA: /ɪmˈpɛnətɹəbəl/, /ɪmˈpɛnɪtɹəbəl/

impenetrable (not comparable)

1. Not penetrable.

   Synonyms: impermeable, impregnable

   Antonyms: penetrable, permeable, pregnable

   The fortress is impenetrable, so it cannot be taken.

   • 2012, John Branch, “Snow fall: The avalanche at Tunnel Creek”, in The New York Times‎^{[https://web.archive.org/web/20240611074920/https://www.nytimes.com/projects/2012/snow-fall/index.html]}, New York, N.Y.: The New York Times Company, →ISSN ↗, →OCLC ↗, archived from the original ↗ on 2024-06-11:

     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. Opaque; obscure; not translucent or transparent.

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

3. (figuratively) Incomprehensible; fathomless; inscrutable.

   Synonyms: unfathomable, Thesaurus:incomprehensible

   Antonyms: fathomable, Thesaurus:comprehensible

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

impenetrable (plural impenetrables)

1. A person not openly given to friendship. (clarification of this definition is needed)
   • 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, […], →OCLC ↗, page 236 ↗:

     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.

• French: impénétrable
• German: undurchlässig
• Portuguese: impenetrável
• Russian: (impermeable) непроница́емый
• Spanish: impenetrable