1. (a.) Incapable of being penetrated or pierced; not admitting the passage of other bodies; not to be entered; impervious; as, an impenetrable shield.

2. (a.) Having the property of preventing any other substance from occupying the same space at the same time.

3. (a.) Inaccessible, as to knowledge, reason, sympathy, etc.; unimpressible; not to be moved by arguments or motives; as, an impenetrable mind, or heart.

Greek to one ambiguous arcane beyond reach beyond understanding cabalistic close close-knit close-textured close-woven closed forever to closed to compact compacted compressed concentrated concrete condensed congested consolidated crammed crammed full crowded dense exuberant firm flourishing gluey gross hard heavy holeproof impassable impermeable impervious impregnable inaccessible inarticulate incognizable incoherent incomprehensible incontestable indomitable inexpugnable inscrutable insuperable invincible inviolable invulnerable irresistible jam-packed jammed jungled jungly lost to lush luxuriant massive mystic nonporous nonpuncturable numinous out of reach overgrown overpowering overrun overwhelming packed past comprehension punctureproof rambling rank resistless riotous serried solid substantial thick thick-growing thickset un-come-at-able unaccessible unaccountable unapproachable unassailable unattainable unavailable unbeatable unconnected unconquerable undiscoverable unfathomable ungettable ungraspable unguessed unintelligible unknowable unobtainable unpassable unperforable unpierceable unprocurable unreachable unsearchable unsecurable unsubduable unsurmountable unweeded unyielding viscid viscose viscous weed-choked weed-ridden weedy