1. Open to doubt or suspicion
   "His doubtable claims about his past made people wary";
   - doubtful, dubious, dubitable, in question, questionable
    
2. [archaic] Inspiring fear
   "the doubtable prospect of major surgery";
   - formidable, redoubtable, unnerving

See also: alarming, deniable, questionable

Encyclopedia: Doubtable