Deuteronomy 19:18

and the judges shall question the witnesses of their times fairly; and, behold, false testimony is in the mouth of the witnesses; they have borne false witness against their brother.

The word chastised here means “accused”. It is also used in the contexts of injury and punishment.

AND THE JUDGES SHALL INQUIRE DILIGENTLY; AND, BEHOLD, IF THE WITNESS BE A FALSE WITNESS. Scripture has not explained how it became known that he is a false witness, for, since the testimony was presented by two witnesses, even if a hundred witnesses were to come and contradict them it would still not be established that they testified falsely [because the testimony of two witnesses carries the same weight as that of a hundred]. Nor can we say that the alleged murder-victim walked [into court], for, in that case Scripture would not say and the judges shall inquire diligently [for an investigation of testimony is incongruous when it is so demonstratively false]. Therefore, trustworthy tradition came and explained that the refutation, and thus nullified, and testimony that is merely contradicted. Ramban holds that if the second pair says, “How can you testify so, when at the same time you say Reuben killed Shimon you yourselves were with us in another place, far removed from that scene,” the first witnesses stand totally refuted and nullified. On the other hand, if the second pair merely contradicts the first pair by claiming that the alleged crime did not take place, we have a case of contradictory witnesses — two against two. If refutation is established and the person on trial has not yet been executed, then the plotting witnesses are subject to the same penalty they wanted to inflict upon their brother. If the verdict has already been executed, see Ramban further in Verse 19. But in the case of contradictory evidence, none of the witnesses is punished since we do not know which one is the false pair (Mishneh Torah, Hilchoth Eiduth 18:2). of witnesses takes place when they [the second pair of witnesses] say to the first “But you were with us on that day [and hour in such a place — away from the alleged scene of the crime].” The reason [that such testimony of the second pair of witnesses is accepted as conclusive] is because their testimony refers to the first witnesses themselves [rather than to their testimony, and the first pair, in order to defend itself, would therefore have to testify as to its own whereabouts at the time of the alleged crime]. Witnesses, however, are not given credence when testifying about themselves saying, “We have not done so,” and the second pair could say of the first pair that they killed someone or that they profaned the Sabbath.

והנה עד שקר העד, שקר ענה באחיו. His testimony was deliberately false, meant to incriminate his fellow Jew. He did not err in what he thought he had witnessed, such as there having been poor lighting on a cloudy day. Neither did he err in the day of the month when the supposed crime occurred, having been unaware that the most recent month had been extended by a day.