Let's look first at the definition of onus, from the Oxford English Dictionary:
A burden; a responsibility or duty. Freq. with the
orig. and chiefly Law. onus of proof n. the obligation to prove an assertion or allegation one makes; the burden of proof
For the Definition #2, you can discharge the onus. From Employment Law, Practical Handbook:
How to discharge the onus of proof in an adverse action claim.......
The Court identified the following as circumstances in which the
employer might not discharge the onus of proof – but ruled that none
of them applied in this case:
the managers’ evidence is discredited in cross-examination;
the managers’ evidence is inherently implausible;
objective facts contradict the managers’ evidence
(You don't want me to quote more, do you? :))
Thus, I suggest that once you have disposed of whatever obligations (legal or otherwise) you assumed when you assumed the onus, you have discharged the onus.
Example (made up): I discharged the onus of providing refreshments
for the after-game party by buying a lot of frozen goodies and
Addendum Provided at Request of the OP:
From Discharging the onus in adverse action claims - What employers can do in the post-Barclay environment: (Partial quote only; see full article for all the riveting facts. Note also that this is from Australia.)
If Mr Crompton engaged in industrial activity, did BHP discharge the
onus of proof?
Justice Collier noted that if she was wrong and that Mr Crompton had
engaged in industrial activity, then BHP Coal bore the onus of proving
that it did not dismiss Mr Crompton for that reason. Her Honour
referred to the decision of Chief Justice French and Justice Crennan
in Barclay, affirming that “the question of why the decision maker [in
an alleged case of adverse action] has acted is one of fact.”
Justice Collier accepted BHP Coal’s submission that it had discharged
that onus, and found that BHP Coal’s decision makers were satisfied
that Mr Crompton had engaged in serious misconduct by “improperly
directing co-workers when to work or not work” and “engaging in
physical and verbal abuse of a co-worker.”
A reason for this conclusion was that Justice Collier preferred the
evidence of BHP Coal’s representatives over Mr Crompton’s testimony.
In choosing to accept the BHP Coal personnel as credible witnesses,
Justice Collier placed considerable emphasis on the thorough internal
investigation conducted by Ms Jorja Roberts (HR Adviser) and Mr Mark
Stroppiana (HR Manager) of BHP Coal
The way I read this, and I am not a lawyer, is that the coal company had the burden of proving it had fired the employee because he had misbehaved, not because he had engaged in industrial activity; the coal company proved it to the satisfaction of the court, and so discharged the onus of proof.