adj. 25B6; adjective
impenetrable armoured plating : UNBREAKABLE, indestructible, solid, thick, unyielding; impregnable, inviolable, unassailable, unpierceable.
a dark, impenetrable forest : IMPASSABLE, unpassable, inaccessible, unnavigable, untraversable; dense, thick, overgrown.
an impenetrable clique : EXCLUSIVE, closed, secretive, secret, private; restrictive, restricted, limited.
impenetrable statistics : INCOMPREHENSIBLE, unfathomable, inexplicable, unintelligible, unclear, baffling, bewildering, puzzling, perplexing, confusing, abstruse, opaque; complex, complicated, difficult; archaic wildering.