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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 WordNetadd/edit