non-starter

noun3

1. •
   Someone or something who was listed to start in a race, but did not start in the race.
2. •
   Something that is destined to fail from the outset; an impractical scheme.

   • Whatever is decided, it is clear that Crossrail 2 must be regarded as a non-starter for the foreseeable future. — 2020
3. •
   An idea or argument that cannot be sensibly debated.