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Definition of arduous : 1. Attended with great labor, like the ascending of acclivities; difficult; laborious; as, an arduous employment, task, or enterprise.
2. Steep and lofty, in a literal sense; hard to climb.

Synonyms:

laboured, intemperate, Augean, challenging, fleshy, cloggy, exacting, straining, wakeless, killing, impenetrable, concentrated, heavy, clayey, easy, threatening, tall, testing, operose, serious, with child, lowering, lumbering, stiff, leaden, persnickety, be an effort, labourious, murderous, herculean, expectant, sweaty, dense, sound, backbreaking, sullen, exhausting, moiling, sonorous, pick-and-shovel, exigent, hellacious, unvoiced, big, weighty, grave, weighed down, effortful, killer, labored, strong, ponderous, gruelling, profound, rigorous, gravid, surd, great, taxing, punishing, voiceless, knockout, large, enceinte, overweight

complicated, formidable, finicky, rough, contrary, cantankerous, harsh, wearying, rugged, laborious, stressful, wearing, difficult, knotty, cumbersome, fussy, toilsome, onerous, tight, demanding, thorny, adverse, strenuous, ticklish, tricky, wearisome, uphill, complex, study atdifficult, tough, grinding, hard

distressful, uncomfortable, anguishing, hurtful, acute, biting, burdensome, inflaming, wrenching, smarting, grievous, odious, irksome, oppressive, offensive, cruel, bothersome, gnawing, aggravating, tormenting, torturing, troublesome, withering, bitter, severe, crushing, annoying, aching, woeful, piercing, grueling, irritating, unbearable, trying, sore, painful, horrendous, besetting, miserable, throbbing, stabbing, cutting, chafing, agonizing, cramping, stinging, unpleasant, disagreeable, excruciating, harrowing, insufferable

Usage examples:

• Upon this arduous undertaking were the troops immediately employed. - "The Campaigns of the British Army at Washington and New Orleans 1814-1815", G. R. Gleig.
• She had learned to go her own way, and sought and found arduous duties in the service of the city and the poor. - "The Complete Historical Romances of Georg Ebers", Georg Ebers.
• But there was the prospect that this search would be long and arduous, and Brandon was not willing to exhaust himself too soon. - "Cord and Creese", James de Mille.