The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            Highest woods impenetrable
            To star or sunlight.                  --Milton.
      [1913 Webster]

   2. (Physics) Having the property of preventing any other
      substance from occupying the same space at the same time.
      [1913 Webster]

   3. Inaccessible, as to knowledge, reason, sympathy, etc.;
      unimpressible; not to be moved by arguments or motives;
      as, an impenetrable mind, or heart.
      [1913 Webster]

            They will be credulous in all affairs of life, but
            impenetrable by a sermon of the gospel. --Jer.
                                                  Taylor.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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, impenetrable]
    3: impossible to understand; "impenetrable jargon"

Moby Thesaurus II by Grady Ward, 1.0:

115 Moby Thesaurus words for "impenetrable":
   Greek to one, ambiguous, arcane, beyond reach,
   beyond understanding, cabalistic, close, close-knit,
   close-textured, close-woven, closed forever to, closed to, compact,
   compacted, compressed, concentrated, concrete, condensed,
   congested, consolidated, crammed, crammed full, crowded, dense,
   exuberant, firm, flourishing, gluey, gross, hard, heavy, holeproof,
   impassable, impermeable, impervious, impregnable, inaccessible,
   inarticulate, incognizable, incoherent, incomprehensible,
   incontestable, indomitable, inexpugnable, inscrutable, insuperable,
   invincible, inviolable, invulnerable, irresistible, jam-packed,
   jammed, jungled, jungly, lost to, lush, luxuriant, massive, mystic,
   nonporous, nonpuncturable, numinous, out of reach, overgrown,
   overpowering, overrun, overwhelming, packed, past comprehension,
   punctureproof, rambling, rank, resistless, riotous, serried, solid,
   substantial, thick, thick-growing, thickset, un-come-at-able,
   unaccessible, unaccountable, unapproachable, unassailable,
   unattainable, unavailable, unbeatable, unconnected, unconquerable,
   undiscoverable, unfathomable, ungettable, ungraspable, unguessed,
   unintelligible, unknowable, unobtainable, unpassable, unperforable,
   unpierceable, unprocurable, unreachable, unsearchable, unsecurable,
   unsubduable, unsurmountable, unweeded, unyielding, viscid, viscose,
   viscous, weed-choked, weed-ridden, weedy