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From The Collaborative International Dictionary of English v.0.48:
grueling \grueling\, gruelling \gruelling\adj. 1. characterized by toilsome effort; requiring extreme physical or mental effort; as, a grueling campaign. Syn: arduous, backbreaking, back-breaking, exhausting, gruelling, hard, heavy, laborious, labourious, punishing, slavish, strenuous, toilsome. [WordNet 1.5]