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Definition of formidable : Exciting fear or apprehension; impressing dread; adapted to excite fear and deter from approach, encounter, or undertaking; alarming.

Synonyms:

shocking, hair-raising, stiff, exacting, terrible, frightening, awesome, imposing, herculean, challenging, fearsome, taxing, rigorous, testing, overwhelming, Augean, fearful, horrifying, redoubted, impressive, redoubtable, alarming, weighty, grueling, impregnable, killer, tremendous, grand, murderous, tall, terrifying, dire, unnerving, sweaty, pick-and-shovel, effortful, horrendous, forbidding, appalling, exigent, moiling, dread, fear, heavy, dangerous, dreadful, ghastly, backbreaking, hellacious, intimidating, frightful, direful, spine-chilling, scary, horrible

fussy, thorny, cumbersome, contrary, strenuous, arduous, laborious, demanding, knotty, rough, uphill, ticklish, tricky, finicky, harsh, grinding, troublesome, difficult, wearing, tough, stressful, wearying, complicated, complex, cantankerous, tight, severe, burdensome, wearisome, hard, adverse, toilsome, onerous, rugged, oppressive, trying, bothersome

dominant, high-powered, commanding, autocratic, important, supreme, influential

muscular, mettlesome, sturdy, stout, stable, energetic, powerful, brawny, mighty, potent, virile, sinewy, robust, firm, vital, strong, vigorous, husky

impervious, indestructible, impenetrable, immune, invincible, unbeatable, invulnerable

Usage examples:

• The return of his formidable army alarmed the fears of the pope, who appears to have equally dreaded the success of his arms abroad and of his presence at home. - "Monks, Popes, and their Political Intrigues", John Alberger.
• My memory of what passed between us is, in some degree, disturbed by the formidable interruption which brought our talk to an end. - "The Legacy of Cain", Wilkie Collins.
• She recognized him as she looked down, and started back with an impulse to escape, he seemed so near and so formidable. - "The Golden Silence", C. N. Williamson and A. M. Williamson.