See arduous in Webster's New World Roget's A-Z Thesaurus II
adjective
Not easy to do, achieve, or master:
difficult, hard, laborious, serious, tall, tough, uphill. See easyRequiring great or extreme bodily, mental, or spiritual strength:
backbreaking, burdensome, demanding, difficult, effortful, exacting, exigent, formidable, hard, heavy, laborious, onerous, oppressive, rigorous, rough, severe, taxing, tough, trying, weighty. See heavy
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