adjective
1.
Synonyms: impassable 1, impermeable, imperviable, impervious, unpierceable
Related Word: firm, solid, substantial
Antonyms: penetrable
2.
Synonyms: incomprehensible 1, incognizable, uncomprehensible, unfathomable, ungraspable, unintelligible, unknowable
Antonyms: penetrable
3.
Synonyms: mysterious , arcane, cabalistic, inscrutable, mysterial, mystic, numinous, unaccountable, unguessed, unknowable