Affidavit on demurrer

What does Affidavit on demurrer mean in American Law?

The definition of Affidavit on demurrer in the law of the United States, as defined by the lexicographer Arthur Leff in his legal dictionary is:

A species of affidavit of good faith which must in some jurisdictions be filed to support a demurrer, stating that the demurrer was not filed merely for delay, but that the defendant believes that he has a good defense on the law, i.e., it is an affidavit of merits addressed specifically to belief in the existence of a legal (as contrasted to a factual) defense.