In a way that is shocking, horrifying, or causing strong disapproval.
Formed from 'appalling' (present participle of appall) + '-ly' (adverbial suffix), creating an adverb that modifies verbs or adjectives.
Adding '-ly' to turn adjectives into adverbs is one of English's most productive word-formation patterns—you can create new adverbs this way even with made-up adjectives, and people will understand you.
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