Source : Webster's Revised Unabridged Dictionary (1913)

Impenetrable \Im*pen"e*tra*ble\, a. [L. impenetrabilis; pref.
   im- not + penetrabilis penetrable: cf. F.
   imp['e]n['e]trable.]
   1. Incapable of being penetrated or pierced; not admitting
      the passage of other bodies; not to be entered;
      impervious; as, an impenetrable shield.

            Highest woods impenetrable To star or sunlight.
                                                  --Milton.

   2. (Physics) Having the property of preventing any other
      substance from occupying the same space at the same time.

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

            They will be credulous in all affairs of life, but
            impenetrable by a sermon of the gospel. --Jer.
                                                  Taylor.

Source : WordNet®

impenetrable
     adj 1: not admitting of penetration or passage into or through; "an
            impenetrable fortress"; "impenetrable rain forests"
            [ant: {penetrable}]
     2: permitting little if any light to pass through because of
        denseness of matter; "dense smoke"; "heavy fog";
        "impenetrable gloom" [syn: {dense}, {heavy}]
     3: impossible to understand; "impenetrable jargon"