Definition of onerous adjective from the Oxford Advanced Learner's Dictionary

 

onerous

 adjective
adjective
BrE BrE//ˈəʊnərəs//
 
; NAmE NAmE//ˈɑːnərəs//
 
, NAmE//ˈoʊnərəs//
 
(formal)
 
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needing great effort; causing trouble or worry synonym taxing an onerous duty/task/responsibility Word Origin late Middle English: from Old French onereus, from Latin onerosus, from onus, oner- ‘burden’.
See the Oxford Advanced American Dictionary entry: onerous