Synonyms for frightful

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1. Causing fright

   fearful, awful, dreadful, horrifying, frightening, terrifying, alarming, scary, horrible, terrible, awesome, awe-inspiring, direful, gruesome, traumatic, ghastly, grisly, macabre, lurid, petrifying, unnerving, hair-raising, bloodcurdling, nightmarish, shocking, repellent, harrowing, appalling, horrific, atrocious, unspeakable, inconceivable, ominous, portentous, fearsome, dismaying, demoralizing, disturbing, disquieting, eerie, weird, uncanny, spooky*, creepy*.

   Antonyms charming*, reassuring, alluring.

2. Very bad or unpleasant

   calamitous, shocking, terrible, horrendous; see offensive 2.