ラテン語のonus, duty, burdenから。PIE*en, burdenから、おそらくPIE*an, on topから、語源的にはonと同じ。
英語の語源
onus (n.)
1640s, from Latin onus "load, burden," figuratively "tax, expense; trouble, difficulty," from PIE *en-es- "burden" (source of Sanskrit anah "cart, wagon"). Hence legal Latin onus probandi (1722), literally "burden of proving."
例文
1. The onus is on employers to follow health and safety laws.
雇用主は健康安全法を遵守する義務がある。
2.The onus is on government departments to show cause why information cannot be disclosed.
政府部門は情報を公開できない理由を説明する責任がある。
3.The onus is on the shopkeeper to provide goods which live up to the quality of their description.
店主は説明に合った品質の商品を提供する義務がある。
4.The onus of proof is reversed in libel actions and placed firmly on the defender.