Merriam-Webster Collegiate Dictionary - onerous

 adjective  Etymology: Middle English, from Middle French honereus, from Latin onerosus, from oner-, onus burden; akin to Sanskrit anas cart  Date: 14th century  1. involving, imposing, or constituting a burden ; troublesome an ~ task  2. having legal obligations that outweigh the advantages an ~ contract  • ~ly adverb  • ~ness noun Synonyms:  ~, burdensome, oppressive, exacting mean imposing hardship. ~ stresses being laborious and heavy especially because distasteful the ~ task of cleaning up the mess. burdensome suggests causing mental as well as physical strain burdensome responsibilities. oppressive implies extreme harshness or severity in what is imposed the oppressive tyranny of a police state. exacting implies rigor or sternness rather than tyranny or injustice in the demands made or in the one demanding an exacting employer.