Adjective

• S: (adj) impenetrable (not admitting of penetration or passage into or through) "an impenetrable fortress"; "impenetrable rain forests"
• S: (adj) dense, heavy, impenetrable (permitting little if any light to pass through because of denseness of matter) "dense smoke"; "heavy fog"; "impenetrable gloom"
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• derivationally related form
• antonym
• S: (adj) impenetrable (impossible to understand) "impenetrable jargon"
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• derivationally related form
• antonym