impenetrable (
http://definr.com/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"