onus

Onus, oneris, n., third declension and an s-stem, so the s of the nominative becomes r between vowels: onus, oneris, oneri, onere, onera, onerum. The same pattern as corpus, tempus and genus, and once you have one you have them all.

Physical cargo comes first, then any obligation, tax or duty, and in legal Latin it is the standard term for a charge attaching to a person or an estate. The verb onero, "to load", gives exonero, "to unload", and English took onerous and exonerate straight from the pair. The neuter plural onera trips people up because it looks like a feminine singular; the agreement will tell you (onera magna, not magna onera in the singular sense).