insuperable

adj

1. Impossible to achieve or overcome or be negotiated.

   “Regular services for passengers and goods were maintained for almost 50 years, but the speed limit of 16 m.p.h., imposed by the Board of Trade, proved an insuperable handicap after the introduction of competitive motor bus services.”

   “While Indian English may not have a uniform standard form throughout the country, this in itself is not an insuperable obstacle for lexicography.”

   “Underground stations were also slated for closure, and there were plans for massive ring roads to cater for the ever-growing numbers of people commuting into town by car. […] However, it soon dawned on politicians that this was unsustainable. Much of London would have had to be demolished to make way for the ring roads, and parking was becoming an insuperable difficulty.”

2. Overwhelming or insurmountable.

   “For the first time I steadily reviewed the obstacles—and to consider them was at once to see they were insuperable.”