What is Per Fraudem?

Legal Definition
A replication to a plea where something has been pleaded which would be a discharge, if it had been honestly pleaded, that such a thing has been obtained by fraud for example, where on debt on a statute, the defendant pleads a prior action depending, if such action has been commenced by fraud the plaintiff may reply per fraudem: 2 Chit. Pl. *675.
-- Bouviers Law Dictionary
Legal Definition
Lat By fraud. Where a plea alleges matter of discharge, and the replication avers that the discharge was fraudulently obtained and is therefore invalid, it is called a "replication per fraudem."
-- Black's Law Dictionary
Legal Definition
By fraud.
-- Ballentine's Law Dictionary