WordWeb Online

1. Characterized by effort to the point of exhaustion; especially physical effort
   "worked their arduous way up the mining valley"
   - backbreaking, grueling [US], gruelling [Brit, Cdn], hard, heavy, laborious, operose, punishing, toilsome
    
2. Taxing to the utmost; testing powers of endurance
   "your willingness after these six arduous days to remain here"
   - straining, strenuous
    
3. Difficult to accomplish; demanding considerable mental effort and skill
   "the arduous work of preparing a dictionary"

See also: arduously, arduousness, difficult, effortful, hard