1. (

• 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" ;

2. (

• 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 ;

3. (

• arduous

) difficult to accomplish; demanding considerable mental effort and skill; "the arduous work of preparing a dictionary" ;