Understanding the etymology of onus is not at all burdensome; it's as simple as knowing that 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"). Additionally, our legal language has onus probandi, which is often shortened to onus. It means "burden of proof"—that is, the obligation of proving a disputed assertion in a court of law.