From WordNet (r) 2.0 (August 2003) [wn]:
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"
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]