In a manner that destroys or causes destruction; in a devastating or ruinous way.
From 'destroying' (present participle of 'destroy') plus the adverbial suffix '-ly,' creating an adverb describing how an action occurs in a destructive manner.
'Destroyingly' is grammatically sound but sounds awkward to native speakers—we'd say 'destructively' instead, showing how language prefers some word formations over others based on rhythm and frequency, not just grammar.
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