not penetrable; that cannot be penetrated, pierced, entered, etc.

inaccessible to ideas, influences, etc.

incapable of being understood; inscrutable; unfathomable

an impenetrable mystery.

Physics . possessing impenetrability.

adjective

clear , penetrable , permeable , porous , soft , thin , comprehensible , intelligible , understandable

adjective

bulletproof , close , compact , firm , hard , hermetic , impassable , impermeable , impervious , inviolable , solid , substantial , thick , unpiercable , arcane , baffling , cabalistic , dark , delphic , enigmatic , enigmatical , hidden , incognizable , indiscernible , inexplicable , inscrutable , mysterious , mystic , obscure , sibylline , unaccountable , unfathomable , ungraspable , unintelligible , unknowable , uncomprehensible , adamant , adamantine , airtight , dense , imperforable , sealed