In a way that is very scary or alarming, or used as an intensifier meaning 'extremely' or 'terribly' (often in British English).
From 'fright' (from Old English 'fyrhto') plus the suffix '-ful' plus '-ly.' The adverbial form became common in the 1800s and is especially popular in British English.
British people have used 'frightfully' as a casual intensifier for centuries—'That's frightfully good'—and it started as a way to be dramatic but ended up as a charming understatement!
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