In a calamitous manner; with terrible consequences or in a disastrous way.
From 'calamitous' + '-ly' (adverb suffix). This creates the adverbial form describing how something happens in a calamitous way.
When events unfold 'calamitously,' they don't just go wrong—they catastrophically spiral, and the word's sound itself (hard 'c's, falling syllables) almost mimics the feeling of disaster unfolding.
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