Definition - What does Zealous Witness mean?
A zealous witness is a witness who is called to testify during a trial who demonstrates a strong bias and enthusiasm for the party that called him or her. Zealous witnesses typically are very anxious about their favored party's fate, and clearly show that they want that party to win the trial.
Justipedia explains Zealous Witness
Zealous witnesses can be a problem in court cases. The reason is because it may be more difficult for a person with a strong bias for one of the parties in the trial to give an accurate and truthful testimony.
Witnesses must swear to tell the truth, the whole truth and nothing but the truth when they are testifying on the witness stand. However, in order to help their favored party win, zealous witnesses may be much more tempted to lie than non-zealous witnesses.