calamitous

1 bringing about ruin or misfortune a calamitous decision to sell their products online exclusively ruined the business Synonyms fatal, cataclysmal (or cataclysmic), catastrophic, damning, destructive, disastrous, fateful, ruinous, unfortunateRelated Words apocalyptic (also apocalyptical); hapless, ill-fated, ill-starred, luckless; adverse, baleful, baneful, damaging, deleterious, detrimental, evil, harmful, hurtful, ill, injurious, noxious, pernicious, prejudicialNear Antonyms fluky (also flukey), fortuitous, fortunate, happy, lucky, providential; auspicious, bright, encouraging, fair, golden, heartening, hopeful, optimistic, promising, propitious, rose-colored, rosy, upbeat

2 causing or tending to cause destruction a calamitous flood that destroyed the town's central business district Synonyms annihilatory, destructive, cataclysmal (or cataclysmic), devastating, devastative, disastrous, ruinousRelated Words antibiotic, biocidal; baleful, deadly, deathly, fatal, fell, killer, lethal, mortal, murderous, pestilent, poisonous, virulent, vital; deleterious, detrimental, harmful, perniciousNear Antonyms preservative, protective; constructive, creative, formative, productive; harmless, innocent, innocuous, inoffensive; ameliorative, helpful, useful; healthful, healthy, nonfatal, nonlethal, salubrious, wholesomeAntonyms nondestructive