In a way that is extremely bad, causing great fear or worry; used to intensify how serious or severe something is.
From 'dreadful' (causing dread or fear, from Old English 'draedan') plus the suffix '-ly' (turning adjectives into adverbs).
British people especially use 'dreadfully' as a very polite understatement—saying something is 'dreadfully sorry' is actually more formal and intense than 'very sorry,' showing how tone and word choice communicate levels of seriousness!
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