A more thorough explanation:

Definition: A rejoinder is a legal term used in common-law pleading. It refers to the defendant's response to the plaintiff's reply or replication.

For example, if a plaintiff files a complaint against a defendant, the defendant may file an answer. If the plaintiff then files a reply to the defendant's answer, the defendant may file a rejoinder in response to the plaintiff's reply.

Here is an example:

Plaintiff: The defendant owes me $10,000 for services rendered.

Defendant: I deny owing the plaintiff any money.

Plaintiff: The defendant did, in fact, agree to pay me for my services.

Defendant: I did not agree to pay the plaintiff for their services, and furthermore, their services were not satisfactory.

Plaintiff: The defendant did agree to pay me, and my services were satisfactory.

Defendant: I deny ever agreeing to pay the plaintiff, and I maintain that their services were unsatisfactory.

In this example, the plaintiff's third statement is a reply to the defendant's second statement. The defendant's fourth statement is a rejoinder to the plaintiff's reply.