In a gruesome manner; in a way that is horrifying, repulsive, or disturbing to behold.
From the adjective 'gruesome' (from 'grue' + '-some', meaning 'causing shuddering') plus the adverbial suffix '-ly'.
The '-some' suffix (like in 'awesome,' 'fearsome,' 'handsome') originally meant 'full of' or 'tending to cause'—so 'gruesome' literally means 'full of shuddering quality,' perfectly capturing why we can't look away from horror!
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