1.  arduous(s = adj.all) backbreaking, grueling, gruelling, hard, heavy, laborious, operose, punishing, toilsome - characterized by effort to the point of exhaustion; especially physical effort; "worked their arduous way up the mining valley"; "a grueling campaign"; "hard labor"; "heavy work"; "heavy going"; "spent many laborious hours on the project"; "set a punishing pace"
Derived form noun arduousness₁

2.  arduous(s = adj.all) straining, strenuous - taxing to the utmost; testing powers of endurance; "his final, straining burst of speed"; "a strenuous task"; "your willingness after these six arduous days to remain here"
Derived form noun arduousness₁

3.  arduous(s = adj.all) - difficult to accomplish; demanding considerable mental effort and skill; "the arduous work of preparing a dictionary"
Derived form noun arduousness₁

1.  Steep and lofty, in a literal sense; hard to climb. [1913 Webster]
"Those arduous paths they trod." [1913 Webster]

2.  Attended with great labor, like the ascending of acclivities; difficult; laborious; as, an arduous employment, task, or enterprise. [1913 Webster]

arduous, adj.
1 hard to achieve or overcome; laborious, strenuous.
2 steep, difficult (an arduous path).

Derivative:
arduously adv. arduousness n.

Etymology:
L arduus steep, difficult

N  difficulty, hardness, impracticability, tough work, hard work, uphill work, hard task, Herculean task, Augean task, task of Sisyphus, Sisyphean labor, tough job, teaser, rasper, dead lift, dilemma, embarrassment, deadlock, perplexity, intricacy, entanglement, complexity, cross fire, awkwardness, delicacy, ticklish card to play, knot, Gordian knot, dignus vindice nodus, net, meshes, maze, coil, crooked path, involvement, nice point, delicate point, subtle point, knotty point, vexed question, vexata quaestio, poser, puzzle, paradox, hard nut to crack, nut to crack, bone to pick, crux, pons asinorum, where the shoe pinches, nonplus, quandary, strait, pass, pinch, pretty pass, stress, brunt, critical situation, crisis, trial, rub, emergency, exigency, scramble, scrape, hobble, slough, quagmire, hot water, hornet's nest, sea of troubles, peck of troubles, pretty kettle of fish, pickle, stew, imbroglio, mess, ado, false position, set fast, stand, standstill, deadlock, dead set, fix, horns of a dilemma, cul de sac, hitch, stumbling block, crab, curmudgeon, difficult, not easy, hard, tough, troublesome, toilsome, irksome, operose, laborious, onerous, arduous, Herculean, formidable, sooner said than done, more easily said than done, easier said than done, difficult to deal with, hard to deal with, ill-conditioned, crabbed, crabby, not to be handled with kid gloves, not made with rose water, awkward, unwieldy, unmanageable, intractable, stubborn, perverse, refractory, plaguy, trying, thorny, rugged, knotted, knotty, invious, pathless, trackless, labyrinthine, intricate, complicated, impracticable, not feasible, desperate, embarrassing, perplexing, delicate, ticklish, critical, beset with difficulties, full of difficulties, surrounded by difficulties, entangled by difficulties, encompassed with difficulties, under a difficulty, in a box, in difficulty, in hot water, in the suds, in a cleft stick, in a fix, in the wrong box, in a scrape, in deep water, in a fine pickle, in extremis, between two stools, between Scylla and Charybdis, surrounded by shoals, surrounded by breakers, surrounded by quicksands, at cross purposes, not out of the wood, reduced to straits, hard pressed, sorely pressed, run hard, pinched, put to it, straitened, hard up, hard put to it, hard set, put to one's shifts, puzzled, at a loss, at the end of one's tether, at the end of one's rope, at one's wit's end, at a nonplus, at a standstill, graveled, nonplused, nonplussed, stranded, aground, stuck fast, set fast, up a tree, at bay, aux abois, driven into a corner, driven from pillar to post, driven to extremity, driven to one's wit's end, driven to the wall, au bout de son Latin, out of one's depth, thrown out, accomplished with difficulty, hard-fought, hard-earned, with difficulty, with much ado, barely, hardly, uphill, against the stream, against the grain, d rebours, invita Minerva, in the teeth of, at a pinch, upon a pinch, at long odds, against long odds, ay there's the rub, hic labor hoc opus, things are come to a pretty pass, ab inconvenienti, ad astra per aspera, acun chemin de fleurs ne conduit a la gloire.