/ Dictionary / Index O onus: Meaning and Definition of Find definitions for: o•nus Pronunciation: (ō'nus), [key] — pl. o•nus•es. a difficult or disagreeable obligation, task, burden, etc. burden of proof. Cf. blame or responsibility. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: onus (Thesaurus) ontology onus probandi* Related Content Daily Word Quiz: immense Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language