Adjective : incontestable proof.    From Dictionary.com. 

Kaplan was struck first by their strict adherence to hierarchy — what he refers to as "the incontestability of command.".    From Wordnik.com.  [In the Line of Fire] Reference

All the elements came together fast, owing to a factor largely missing from civilian life: the incontestability of command.    From Wordnik.com.  [Five Days in Fallujah] Reference

Some courts treat incontestability as creating a rebuttable presumption of strength, but the analysis should be separate: “there is no apparent reason that the incontestable status of a mark should automatically translate into a well-known mark.”.    From Wordnik.com.  [Archive 2009-04-01] Reference

Some courts treat incontestability as creating a rebuttable presumption of strength, but the analysis should be separate: "there is no apparent reason that the incontestable status of a mark should automatically translate into a well-known mark.".    From Wordnik.com.  [Rebecca Tushnet's 43(B)log] Reference

If only the Lanham Act didn’t provide that incontestability is no defense to a violation of §2a, which prohibits the registration of deceptive marks.    From Wordnik.com.  [Nature Made makes unnaturally bad removal argument] Reference

Side note: trademark really gets confused when it comes to incontestability of a descriptive term that was registered in a particular form, like a particular font — do you focus on the similarity to the mark as specifically displayed, or do you look at the similarity of the word component of the mark, which it’s possible to abstract from the mark?.    From Wordnik.com.  [Archive 2009-08-01] Reference

(Though I think this is a descriptive/nominative use, there is arguably a doctrinal reason to stick with descriptiveness: some of LEYE’s marks are incontestable and nominative fair use is not a specifically listed defense to incontestability; however, courts have used nominative fair use to find no confusion as a matter of law, so that’s not a huge barrier to applying the doctrine to incontestable marks.).    From Wordnik.com.  [When is descriptive use not descriptive?] Reference