adjective
imposing great hardship or strain
found the care of his old mother an onerous task
Synonyms: burdensome, demanding, exacting, exigent, grievous, oppressive, superincumbent, taxing, tough, trying, weighty
Related Word: arduous, difficult, hard, laborious; heavy, hefty, ponderous; cumbersome, unruly, unwieldy; driving, heavy-handed
Contrasted words: easy, effortless; facile, light, simple, smooth, unexacting, untaxing