Source : Webster's Revised Unabridged Dictionary (1913)

Unquestionable \Un*ques"tion*a*ble\, a.
   1. Not questionable; as, an unquestionable title.

   2. Not inviting questions or conversation. [R.] --Shak. --
      {Un*ques"tion*a*bly}, adv.

Source : WordNet®

unquestionable
     adj 1: impossible to question; "unquestionable authority" [ant: {questionable}]
     2: not counterfeit or copied; "an authentic signature"; "a bona
        fide manuscript"; "an unquestionable antique";
        "photographs taken in a veritable bull ring" [syn: {authentic},
         {bona fide}, {veritable}]
     3: not open to question; "an unquestionable (or unequivocal)
        loss of prestige"