Synonyms for Arduous:

adj.

all (adjective)
grueling, laborious, punishing, heavy, backbreaking, toilsome, hard, gruelling.
difficult (adjective)
strenuous, onerous, adverse, severe, knotty, troublesome, wearing, fussy, hard, rough, study atdifficult, thorny, laborious, tricky, ticklish, harsh, uphill, rugged, stressful, bothersome, tight, wearying, oppressive, formidable, cantankerous, burdensome, cumbersome, trying, demanding, toilsome, complicated, difficult, tough, wearisome, complex, finicky, grinding, contrary.
difficult, hard to endure (adjective)
harsh, painful, tough, onerous, heavy, troublesome, toilsome, rough, backbreaking, grueling, taxing, punishing, burdensome, formidable, strenuous, uphill, rigorous, trying, laborious, severe.
effortful (adjective)
effortful.
painful (adjective)
cruel, agonizing, harrowing, unbearable, painful, woeful, withering, severe, troublesome, inflaming, disagreeable, Besetting, chafing, oppressive, annoying, Torturing., miserable, throbbing, smarting, excruciating, horrendous, bothersome, Cramping, insufferable, unpleasant, stinging, sore, crushing, aching, Irksome, grievous, biting, hurtful, distressful, grueling, uncomfortable, trying, piercing, offensive, odious, irritating, aggravating, anguishing, bitter, cutting, Gnawing, acute, tormenting, burdensome, wrenching, stabbing.
severe (adjective)
severe.

n.

be an effort. exigent, rigorous, exacting, taxing. arduous (noun)
punishing, strenuous, hard, laborious, labourious, straining, gruelling, heavy, difficult, toilsome, effortful, backbreaking, grueling.

Other synonyms:

tall. exigent, rigorous. exacting, taxing. serious. Other relevant words:
serious, tall, heavy, punishing, rigorous, straining, gruelling, labourious, effortful, taxing, exacting, backbreaking, exigent.

Usage examples for arduous

  1. Graham, devoted such arduous labour. – Five of Maxwell's Papers by James Clerk Maxwell
  2. But the arduous duties are over for the season. – One of Our Conquerors, v2 by George Meredith Edition: 10 Language: English
  3. It was an arduous journey, taking him well south of Lang's; but it was evidently successful. – Roosevelt in the Bad Lands by Hermann Hagedorn