on·er·ous (?n??r-?s, ??n?r-)adj.1. Troublesome or oppressive; burdensome. See Synonyms at burdensome.2. Law Entailing more liabilities than benefits or imposing significant obligations.[Middle English, from Old French onereus, from Latin oner?sus, from onus, oner-, burden.]on?er·ous·ly adv.on?er·ous·ness n.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.