arduous, 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")
arduous, 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"- F.D.Roosevelt)
arduous -- (difficult to accomplish; demanding considerable mental effort and skill; "the arduous work of preparing a dictionary")
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