In a manner that causes fright or terror; in a way that is frightening or alarming.
From 'affright' (to frighten greatly) plus the '-ingly' suffix. 'Affright' combines 'a-' with 'fright,' intensifying the meaning of the base word to suggest sudden, extreme fear.
This adverb appears mostly in 17th and 18th-century literature—as modern English simplified its adverb formations, 'affrightingly' fell out of use in favor of simpler forms like 'frighteningly,' showing how language gradually streamlines itself.
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