E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The writ of discharge formerly granted to those barons and knights who personally attended the king on a foreign expedition. At their discharge they were exempt from the claim of scutage or knights fee. Subsequently the term was applied to the acquittance which a sheriff receives on settling his account at the Exchequer; and, later still, to any discharge of an account: thus Webster says
You had the trick in audit-time to be sick till I had signed your quietus.Duchess of Malfy (1623).
Quietus. A severe blow; a settler; death, or discharge from life.