WordWeb Online

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

See also: dense, incomprehensible, thick, uncomprehensible

Antonym: penetrable

Encyclopedia: Impenetrable