Very bad, unpleasant, or shocking. It can describe something that causes great fear, suffering, or disappointment.
From 'dread' + '-ful', formed in Middle English. 'Dread' comes from Old English roots meaning 'fear'. The adjective originally meant 'full of dread' or 'inspiring dread'.
Over time, 'dreadful' softened from 'inspiring terror' to also covering everyday complaints, like 'a dreadful meal'. English often takes words for intense emotions and uses them casually. That slide in meaning can hide how powerful the original feeling was.
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