frightful in WordNet English dictionary

1. provoking horror
   "an atrocious automobile accident"; "a frightful crime of decapitation"; "an alarming, even horrifying, picture"; "war is beyond all words horrible"- Winston Churchill; "an ugly wound"
   atrocious  frightful  horrifying  horrible  ugly 
   
2. extreme in degree or extent or amount or impact
   "in a frightful hurry"; "spent a frightful amount of money"
   frightful  terrible  awful  tremendous 
   
3. extremely distressing
   "fearful slum conditions"; "a frightful mistake"
   fearful  frightful