A more thorough explanation:

Fictitious issue is a legal term that refers to a proceeding where the parties agree to have an issue tried by a jury without actually bringing a formal action. This is usually done when a court lacks jurisdiction or is unwilling to decide the issue.

An example of a fictitious issue is when a court directs a matter to be tried by a jury at the bar of the court of king's bench or at the assises, upon a feigned issue. For instance, if a matter of fact is strongly controverted, the court may not bind the parties by written depositions but instead directs the matter to be tried by a jury. In this case, an action is feigned to be brought, wherein the pretended plaintiff declares that he laid a wager of 5l. with the defendant that A was heir at law to B; and then avers that he is so; and therefore demands the 5l. The defendant allows the wager, but avers that A is not the heir to B; and thereupon the issue is joined.

This example illustrates how a fictitious issue is used to determine some disputed rights without the formality of pleading.