Synonyms for Frightful (same or very similar meaning)
WordNet sense 1 (extreme in degree or extent or amount or impact):
awful, terrible, tremendous
WordNet sense 2 (beyond what is ordinary or usual; highly unusual or exceptional or remarkable):
extraordinary
WordNet sense 3 (extremely distressing):
fearful
WordNet sense 4 (having undesirable or negative qualities):
bad
WordNet sense 5 (provoking horror):
atrocious, horrible, horrifying, ugly
WordNet sense 6 (causing alarm or fear):
alarming
From the ODE community, based on WordNet
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