In a grewsome or gruesome manner; in a way that is disturbing and horrifying.
Formed from 'grewsome' plus the adverbial suffix '-ly,' following English word-building patterns to create an adverb from an adjective. The same formation applies to the modern 'gruesome' → 'gruesomely.'
Adding '-ly' to turn adjectives into adverbs is one of English's most productive word-making tools—it's so common that we can instantly understand new adverbs like this, even if we've never heard them before.
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