undownable

adjective2

1. •
   (rare) Invincible; which cannot be brought down or overcome.

   • The SR-71 is claimed to be undownable by virtue of its altitude and speed. — 1998
2. •
   (rare) Undeniably important; which cannot be played down or ignored.

   • If ever there was an example of sheer pluck and undownable determination it is right here[…] — 1923