1.Definition: extreme in degree or extent or amount or impact

"In a frightful hurry", "Spent a frightful amount of money"

Related Adjective(s):awful, terrible, tremendous

2.Definition: extremely distressing

"A frightful mistake"

Related Adjective(s):fearful

3.Definition: provoking horror

"A frightful crime of decapitation"

Related Adjective(s):atrocious, horrible, horrifying, ugly