The noun onus means “a disagreeable necessity”, an “obligation”, “burden”, or “blame”. From Merriam-Webster:

English borrowed the word — spelling, meaning, and all — from Latin in the 17th century. We can also add that it’s a distant relative of the Sanskrit word for “cart” (a vehicle that carries a burden). English isn’t exactly loaded with derivatives of Latin “onus,” but the root did give us “onerous” (“troublesome”) and “exonerate” (“to clear from accusation or blame” — thus, “to unburden”).

The word also appears in the legal term onus probandi, burden of proof.