This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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adj. Demanding great effort or labor; difficult: "the arduous work of preparing a Dictionary of the English Language” ( Thomas Macaulay).

adj. Testing severely the powers of endurance; strenuous: a long, arduous, and exhausting war.

adj. Hard to traverse, climb, or surmount. See Synonyms at burdensome, hard.

adj. Needing or using up much energy; testing powers of endurance.

adj. burning; ardent

adj. this sense) Difficult or exhausting to traverse.

adj. Steep and lofty, in a literal sense; hard to climb.

adj. Attended with great labor, like the ascending of acclivities; difficult; laborious.

Steep, and therefore difficult of ascent; hard to climb.

Attended with great labor, like the ascent of acclivities; difficult.

Hence—3. Energetic; laborious: said of persons or actions: as, an arduous student;

adj. characterized by effort to the point of exhaustion; especially physical effort

adj. difficult to accomplish; demanding considerable mental effort and skill

adj. taxing to the utmost; testing powers of endurance

"U.S. power officials now say this outage is contained and what they call the arduous process of restoring systems is underway and we have gotten some confirmation of that from our reporters in New York City."

effortful    difficult    hard   

laborious    painful    difficult    trying    exhausting    difficulty    demanding    burdensome   

strenuous    hazardous    laborious    toilsome    fruitless    tedious    protracted    difficult    irksome    grueling