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Adjective: impenetrable  im'pe-ni-tru-bul
  1. Not admitting of penetration or passage into or through
    "an impenetrable fortress"; "impenetrable rain forests"
     
  2. Permitting little if any light to pass through because of denseness of matter
    "impenetrable gloom";
    - dense, heavy
     
  3. Impossible to understand
    "impenetrable jargon"

See also: dense, incomprehensible, thick, uncomprehensible

Antonym: penetrable

Encyclopedia: Impenetrable