1.
Frightful means very bad or unpleasant. (OLD-FASHIONED)
My father was unable to talk about the war, it was so ~.
= terrible
ADJ
2.
Frightful is used to emphasize the extent or degree of something, usually something bad. (INFORMAL, OLD-FASHIONED)
He got himself into a ~ muddle...
= dreadful
ADJ: ADJ n emphasis