undownable

un- +‎ downable

• (UK, US) IPA^(key): /ʌnˈdaʊn.ə.bəl/

undownable (not comparable)

1. (rare) Undeniably important; which cannot be played down or ignored.
   • 1923, Interstate Cotton Seed Crushers' Association, Official Monthly Bulletin:

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

   • 1930, Charles J. V. Murphy, Parachute:

     There is the undownable legend that […]

   • 1998, Frank N. Riale, Sinless, Sickless, Deathless Life:

     […] the truth which came home with such undownable conviction […]

2. (rare, neologism) Invincible; which cannot be brought down or overcome.
   • 1998, Andrew Pavacic, “Why are our ASMs smaller than theirs?”, in sci.military.naval, Usenet‎:

     The SR-71 is claimed to be undownable by virtue of its altitude and speed.

   • 1998, Edmond Wollmann, “A Challenge to Lazzwaldo AKA anonym or whatever it is”, in sci.skeptic, Usenet‎:

     […] this paranoic attempt to discredit the credible, and down the undownable.

   • 1999, kinara, “Taiwan is NOT able to defeat any invasion”, in soc.culture.china, Usenet‎:

     I did not go as far as that either. I only went as far as stating that an American warplane is almost undownable and an American warship is unsinkable.