Causing fright or fear; frightening, alarming, or terrifying in nature.
From 'affright' + the adjectival suffix '-ful,' meaning 'full of' or 'causing' fright. The form is archaic and rarely used.
English once had many '-ful' adjectives based on verbs, but most disappeared—we abandoned 'affrightful' in favor of just 'frightful,' showing how English constantly streamlines its vocabulary.
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