A more thorough explanation:

A failure-of-proof defense is when a party argues that the other party has not provided enough evidence to prove an essential fact in their claim or defense.

For example, in a criminal trial, the prosecution must prove beyond a reasonable doubt that the defendant committed the crime. If the prosecution fails to provide enough evidence to prove the defendant's guilt, the defense can use a failure-of-proof defense to argue that the prosecution has not met its burden of proof.

Another example could be in a civil lawsuit where the plaintiff is suing the defendant for breach of contract. The defendant can use a failure-of-proof defense to argue that the plaintiff has not provided enough evidence to prove that a contract existed between the two parties.

These examples illustrate how a failure-of-proof defense can be used to challenge the other party's evidence and weaken their claim or defense.