impassible, impassable – Impassible is incapable of feeling or suffering; impassable is not capable of being passed. More…
impregnable – Means “incapable of being taken by force” (from French prendre, “seize”). More…
indefatigable – Means “incapable of being wearied.” More…
inimitable – Means “unique; incapable of being imitated.” More…