formidable

1 causing fear a formidable, irascible old man who frightened the neighborhood children Synonyms alarming, dire, direful, dread, dreadful, fearsome, forbidding, fearful, frightening, frightful, ghastly, hair-raising, horrendous, horrible, horrifying, intimidating, redoubtable, scary, shocking, spine-chilling, terrible, terrifyingRelated Words daunting, demoralizing, disconcerting, discouraging, dismaying, disquieting, distressing, disturbing, perturbing, startling, threatening, troubling, trying, unnerving; bone-chilling, creepy, eerie (also eery), weird; appalling, atrocious, awful, grisly, gruesome (also grewsome), hideous, horrid, macabre, monstrous, nightmarishNear Antonyms calming, comforting, consoling, inviting, lulling, pacifying, quieting, reassuring, relaxing, soothing, tranquilizing (also tranquillizing); nonintimidating, nonthreatening

2 requiring considerable physical or mental effort running a marathon is a formidable undertaking Synonyms arduous, Augean, backbreaking, challenging, demanding, difficult, effortful, exacting, hard, grueling (or gruelling), heavy, hellacious, herculean, killer, 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