WordWeb Online

1. Provoking horror
   "a frightful crime of decapitation"
   - atrocious, horrifying, horrible, ugly
    
2. Extreme in degree or extent or amount or impact
   "in a frightful hurry"; "spent a frightful amount of money"
   - terrible, awful, tremendous
    
3. Extremely distressing
   "a frightful mistake"
   - fearful

See also: alarming, bad, extraordinary, frightfulness

Encyclopedia: Frightful