adjective
1.
not allowing passage
an impassable barrier
Synonyms: impenetrable, impermeable, imperviable, impervious, unpierceable
Contrasted words: penetrable, permeable, pervious
Antonyms: passable
2.
Synonyms: insuperable , inconquerable, indomitable, insurmountable, invincible, unconquerable, unsurmountable