Impenetrable a. : Incapable of being penetrated or pierced; not admitting the passage of other bodies; not to be entered; impervious; as, an impenetrable shield.

Impenetrable a. : Having the property of preventing any other substance from occupying the same space at the same time.

Impenetrable a. : Inaccessible, as to knowledge, reason, sympathy, etc.; unimpressible; not to be moved by arguments or motives; as, an impenetrable mind, or heart.

Example:
when are companies going to invent Impenetrable tires?

Taxonomy
Merriam-Webster Online Dictionary, Dictionary, webster, pen, Word Games, Webster Online Dictionary with audio pronunciations, Middle English, thesaurus, Google Search, Custom Search, glossary, language, ESL, Search, ble, adjective, Etymology, Middle French.

Defination from the web:
impenetrable from the Merriam-Webster Online Dictionary with audio pronunciations, thesaurus, Word of the Day, and word games. ... Etymology: Middle English impenetrabel, from Middle French impenetrable, from Latin impenetrabilis, from in- + penetrabilis penetrable ...