Augustine of Hippo

In the Decalogue itself it is written, "You shall not bear false witness," in which classification every lie is embraced, for whoever pronounces any statement gives testimony to his own mind. If anyone should argue that not every lie should be called false witness, what will he answer to this statement which is also in the sacred Scriptures: "The mouth that belies, kills the soul"? …

On Lying 5.6 ↗

Thomas Aquinas

The Lord has forbidden anyone to injure his neighbor by deed; now he forbids us to injure him by word. "You shall not bear false witness against your neighbor." This may occur in two ways, either in a court of justice or in ordinary conversation. In the court of justice it may happen in three ways, according to the three persons who may violate this Commandment in court. The first person is the plaintiff who makes a false accusation: "You shall not be a detractor nor a whisperer among the people" [Lev 19:16]. …

Explanation of the Ten Commandments ↗

Bonaventure

Here is prohibited the lie that is harmful to one's neighbor. And according to the general understanding, every pernicious lie is here prohibited: and it is to be understood that every lie is evil. Augustine distinguishes and says that there is the officious, jocose, and pernicious lie. A lie is called officious that harms no one and benefits someone. The jocose lie is one that is made not with the will to harm but with the desire to lie, and sometimes it harms, but does not benefit. …

Collationes de Decem Praeceptis, Collation 7 ↗