- impenetrable - not admitting of penetration or passage into or through; "an impenetrable fortress"; "impenetrable rain forests"
Antonym: penetrable
- dense, heavy, impenetrable - permitting little if any light to pass through because of denseness of matter; "dense smoke"; "heavy fog"; "impenetrable gloom"
Antonym: thin (indirect, via thick)
- impenetrable - impossible to understand; "impenetrable jargon"
Antonyms: comprehensible, comprehendible (indirect, via incomprehensible)