frightful (adj.)

mid-13c., "timid, fearful, full of terror," from fright (n.) + -ful. The prevailing modern sense of "alarming, full of occasion for fright" is from c. 1600. Meaning "dreadful, horrible, shocking" (often hyperbolic) is attested from c. 1700; Johnson noted it as "a cant word among women for anything unpleasing." Related: Frightfully; frightfulness. Middle English also had frighty "causing fear," also "afraid" (mid-13c.).

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Definitions of frightful from WordNet

frightful (adj.)
provoking horror; "war is beyond all words horrible"- Winston Churchill;
a frightful crime of decapitation
Synonyms: atrocious / horrifying / horrible / ugly
frightful (adj.)
extreme in degree or extent or amount or impact;
spent a frightful amount of money
in a frightful hurry
Synonyms: terrible / awful / tremendous
frightful (adj.)
extremely distressing;
a frightful mistake
Synonyms: fearful