A more thorough explanation:

Definition: Quominus (kwoh-muh-nuhs or kwoh-mI-nuhs) is a Latin term used in a 14th-century Exchequer writ. It alleges that the plaintiff had lent the defendant a sum of money and that the plaintiff was unable to repay a debt of similar amount to the Crown because of the debt to the defendant. In other words, the plaintiff pleaded the fiction that he was a debtor of the king who could not repay that debt because of the defendant's failure to repay him.

Example: Let's say that John borrowed $100 from Mary and promised to pay her back in a month. However, John also owed $100 to the government as taxes. When the time came to pay Mary back, John didn't have the money because he had to pay his taxes. So, Mary sued John using the writ of quominus, claiming that she was unable to repay her own debt to the government because of John's debt to her.

Explanation: The example illustrates how the writ of quominus was used as a means of collecting the royal revenue. It allowed citizens to sue in the court of the king in order to collect a private debt. The plaintiff had to allege that he was 'less able' to pay the king when his debtors would not pay him. This fiction was used to keep up the pretense that the matter concerned the royal revenue. However, everyone, even the court itself, recognized this as a mere fiction, and that since the suit was in fact between the plaintiff and the defendant, the defendant was not permitted to bring in other matters, such as a defense on the ground that the plaintiff did not actually owe any taxes to the crown.