on ‧ er ‧ ous /ˈɒnərəs, ˈəʊ- $ ˈɑː-, ˈoʊ-/ BrE AmE adjective formal
[ Date: 1300-1400 ; Language: Old French ; Origin: onereus , from Latin onerosus , from onus ; ⇨ ↑ onus ]
work or a responsibility that is onerous is difficult and worrying or makes you tired ⇨ burdensome :
an onerous task