A more thorough explanation:

A zealous witness is a person who gives testimony in a way that shows they strongly support one side of a case. This can happen when a witness is overly enthusiastic or passionate about their testimony, making it clear that they are biased towards one party.

For example, if a witness is testifying in a criminal trial and they are a close friend of the defendant, they may be a zealous witness. They may be so passionate about defending their friend that they exaggerate or distort the truth to make their friend look innocent.

Another example could be a witness who is testifying in a civil lawsuit and they are an employee of the company being sued. They may be a zealous witness because they want to protect their employer and keep their job, so they may exaggerate or downplay certain facts to help their employer win the case.

These examples illustrate how a zealous witness can be a problem in a legal case because their testimony may not be completely truthful or objective. It is important for witnesses to be honest and impartial when giving testimony so that the court can make a fair and just decision.