WordWeb Online

1. Incapable of being questioned
   "unquestionable authority"
    
2. Not counterfeit or copied
   "an unquestionable antique"
   - authentic, bona fide, veritable
    
3. Not open to question
   "an unquestionable (or unequivocal) loss of prestige"

See also: acknowledged, beyond doubt, echt, for sure, genuine, incontestable, incontestible, indubitable, mathematical, unambiguous, undeniable, unequivocal, unimpeachable, univocal, unquestionableness, unquestionably

Antonym: questionable