A responsibility or obligation
A burdensome personal duty or task
A legally mandated responsibility or requirement
A burden of guilt, shame, or blame
A burden of proof
We all have burdens to bear, and most of those burdens could probably be referred to as onuses. Now this doesn't mean that, if you were carrying a heavy bag of potatoes, we would call that bag an onus, since we don't use onus to speak of physical weights these days. However, we do use onus to refer to unpleasant tasks which we are required to perform by duty or by law. Thus, you might describe your obligation to carry potatoes around every day, more so than the potatoes themselves, as "a heavy onus." Similarly, if the law required you to carry a sack of potatoes at least once a day, that would also be an onus. In this way, onus refers to a burden of responsibility. And if you were accused of failing to perform your legal obligation to carry potatoes around in a sack, then, in a legal system where one is presumed innocent until proven guilty, the onus of proof, i.e. the burden or responsibility of proving, is on the authorities to show that you have been negligent in your potato duties.
Onus can also refer to burdens of guilt, shame, or blame. For example, if you borrowed your friend's textbook and then lost it, you would bear a heavy onus of guilt even if your crime was never discovered. And, if your crime were discovered, you might feel that you then bore a similar onus of shame. Finally, if you were falsely accused of losing your friend's textbook when you actually didn't (the most likely outcome, since you would never do something like that) you would still bear an onus, this time one of blame rather than of shame or guilt.
Example: Atlas was condemned by Zeus to forever hold the onus of the Heavens upon his shoulders.
Example: A parent should view their parental responsibility not as an onus but as a privilege.
Example: Too often, children feel as if they bear the onus for their parents' separation.
Onus first entered English in the mid-17th century. It derives directly from the Latin onus which means literally a burden or figuratively a tax, a trouble, or a difficulty. This word stems from the Proto-Indo European root en-es meaning "burden."
Onerous: This adjective form describes a task, duty, or obligation which is burdensome.
Example: To young Charles taking out the trash seemed an insufferably onerous chore.
From Miss Thackeray's Old Kensington:
Mr. Tapeall recommended that his clients should do nothing for the present. The onus of proof lay with the opposite side. Mr. Raban had promised to ascertain all particulars, as far as might be: on his return from the Crimea they would be in a better position to judge.
Here, Thakeray uses onus in a legal context to describe the responsibility for proving a claim which is placed on those attempting to contest the validity of a will.
An onus is a burden placed on us.
Bring out the linguist in you! What is your own interpretation of onus. Did you use onus in a game? Provide an example sentence or a literary quote.