Adjective arduous has 3 senses
  1. arduous, backbreaking, grueling, gruelling, hard, heavy, laborious, punishing, toilsome - characterized by toilsome 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"
    Antonym: effortless (indirect, via effortful)
  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
    Antonym: effortless (indirect, via effortful)
  3. arduous - difficult to accomplish; demanding considerable mental effort and skill; "the arduous work of preparing a dictionary"
    easy (indirect, via difficult)