killer

1 likely to cause or capable of causing death killer viruses that claimed millions of lives Synonyms baleful, deathly, fatal, fell, deadly, 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

2 requiring considerable physical or mental effort a killer exercise program guaranteed to whip you into shape Synonyms arduous, Augean, backbreaking, challenging, demanding, difficult, effortful, exacting, formidable, grueling (or gruelling), heavy, hellacious, herculean, hard, laborious, moiling, murderous, pick-and-shovel, rigorous, rough, rugged, severe, stiff, strenuous, sweaty, tall, testing, toilsome, tough, uphillRelated Words abstract, abstruse, complex, complicated, elusive, hairy, insoluble, intricate, involved, knotty, opaque, problematic (also problematical), recondite, serious, spiny, stubborn, thorny, ticklish, tricky; bruising, burdensome, exhausting, labored, onerous, oppressive, stressful, taxing, tight, trying; annoying, bothersome, distressing, irksome, troublesome, vexatious; grievous, grim, strict, stringent; brutal, cruel, inhuman, painfulNear Antonyms achievable, clear, doable, elementary, manageable, uncomplicated; comforting, gentle, painless, relaxed, smooth, soothing; accessible, friendly, idiotproof, user-friendlyAntonyms cheap, easy, effortless, facile, light, mindless, simple, soft, undemanding