Arduous, a.
Etymology: [L. arduus steep, high; akin to Ir. ard high, height.]
1. Steep and lofty, in a literal sense; hard to climb. Those arduous pats they trod. Pope.
2. Attended with great labor, like the ascending of acclivities; difficult; laborious; as, an arduous employment, task, or enterprise.
Syn. -- Difficult; trying; laborious; painful; exhausting. -- Arduous, Hard, Difficult. Hard is simpler, blunter, and more general in sense than difficult; as, a hard duty to perform, hard work, a hard task, one which requires much bodily effort and perseverance to do. Difficult commonly implies more skill and sagacity than hard, as when there is disproportion between the means and the end. A work may be hard but not difficult. We call a thing arduous when it requires strenuous and persevering exertion, like that of one who is climbing a precipice; as, an arduous task, an arduous duty. "It is often difficult to control our feelings; it is still harder to subdue our will; but it is an arduous undertaking to control the unruly and contending will of others."
Shades of meaning
Fernald has no entry of its own for arduous; he lists it under difficult.
Where he writes about it:
Arduous (L. arduus, steep) signifies primarily so steep and lofty as to be difficult of ascent, and hence applies to that which involves great and sustained exertion and ordinarily for a lofty aim; great learning can only be won by arduous toil. — from difficult
Arduous is always active. — from difficult