The Oxford Thesaurus An A-Z Dictionary of Synonyms - arduous adj.

1) laborious, difficult, hard, tough, strenuous, onerous, burdensome, back-breaking, painful, Formal operose; tiring, exhausting, wearisome, fatiguing, taxing, gruelling, trying, formidable: The Sherpas were well equipped for the arduous climb. What an arduous task it is to read the proofs of a dictionary! 2 energetic, strenuous, vigorous: Montrose made arduous efforts to reconstruct his army.