impenetrable

adj. incapable of being penetrated or entered; unsusceptible to outside ideas or influences; unfathomable, inscrutable

In metaphysics, impenetrability is the name given to that quality of matter whereby two bodies cannot occupy the same space at the same time. The philosopher John Toland argued that impenetrability and extension were sufficient to define matter, a contention strongly disputed by Gottfried Wilhelm Von Leibniz.
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Adjective
1. not admitting of penetration or passage into or through; "an impenetrable fortress"; "impenetrable rain forests"
(antonym) penetrable
(similar) dense, thick
2. permitting little if any light to pass through because of denseness of matter; "dense smoke"; "heavy fog"; "impenetrable gloom"
(synonym) dense, heavy
(similar) thick
3. impossible to understand; "impenetrable jargon"
(similar) incomprehensible, uncomprehensible

adj. impenetrable, inscrutable, fathomless

adj. impervious, impenetrable, inscrutable