onus (n.)youdaoicibaDictYouDict[onus 词源字典]
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."[onus etymology, onus origin, 英语词源]