fatal

1 bringing about ruin or misfortune I made the fatal mistake of sharing my secret with the office's biggest blabbermouth Synonyms calamitous, cataclysmal (or cataclysmic), catastrophic, damning, destructive, disastrous, fateful, ruinous, unfortunate Related Words apocalyptic (also apocalyptical); hapless, ill-fated, ill-starred, luckless; adverse, baleful, baneful, damaging, deleterious, detrimental, evil, harmful, hurtful, ill, injurious, noxious, pernicious, prejudicial Near Antonyms fluky (also flukey), fortuitous, fortunate, happy, lucky, providential; auspicious, bright, encouraging, fair, golden, heartening, hopeful, optimistic, promising, propitious, rose-colored, rosy, upbeat

2 likely to cause or capable of causing death that snake's venom is fatal unless the victim is given the antidote almost immediately Synonyms baleful, deathly, deadly, fell, killer, lethal, mortal, murderous, pestilent, terminal, vitalRelated Words baneful, deleterious, destructive, harmful, injurious, noxious, pernicious, truculent; infectious, infective, pestilential, poisonous, sublethal, toxic, virulent; dangerous, grave, grievous, hazardous, jeopardizing, menacing, parlous, perilous, risky, serious, threatening, ugly, unhealthy, unsound; bloody, internecine, sanguinary, sanguineNear Antonyms beneficial, restorative, salubrious, salutary; alleviative, corrective, curative, remedial, tonic; advantageous, beneficial, useful; nonpoisonous, nontoxic, safeAntonyms healthful, healthy, nonfatal, nonlethal, wholesome