What does Luke 23:9 mean?

Jewish religious leaders insist Jesus deserves death for a crime against Rome. Pilate knows they're lying (Matthew 27:18). But he's on good terms with the high priest Caiaphas; he keeps him installed as high priest for almost twenty years. He doesn't want to antagonize the Sanhedrin, which is far more a threat to civic peace than Jesus. So, he tries to find an out (Luke 23:1–5).

Herod the Great and his descendants who remain in Jewish territory are Edomite and Arabic. But culturally and religiously, they're Jewish, at least nominally. That means Herod Antipas, the tetrarch of Galilee, is in Jerusalem to celebrate Passover. When Pilate learns from the Jewish leaders that Jesus is from Galilee, he sends Jesus to the highest-ranking "Jew" in the city (Luke 23:6–7).

Antipas is elated. He has heard about Jesus and even worried that Jesus is John the Baptist raised from the dead (Mark 6:14–16). He both wants to kill Jesus (Luke 13:31) and see what is causing so much commotion. He's hoping to witness some of the miracles people have been claiming Jesus performs (Luke 23:8).

Antipas spends a significant amount of time questioning Jesus, but he's disappointed. Jesus won't perform for him. He won't even speak. In that way, Jesus is fulfilling part of the prophecy of the Suffering Servant:

He was oppressed, and he was afflicted,
yet he opened not his mouth;
like a lamb that is led to the slaughter,
and like a sheep that before its shearers is silent,
so he opened not his mouth (Isaiah 53:7).

Scholars speculate that Jesus refuses to speak to Antipas because Pilate has already called Him innocent (Luke 23:4). That would make further questioning a sign that the trial is unjust. But a pattern in the Gospels reveals something additional.

Jesus speaks to identify Himself. When Jesus is before the chief priests, scribes, and elders, He identifies Himself as the Christ, the Son of God (Matthew 26:63–64; Mark 14:61–62; Luke 22:67–70). In the same way, Jesus says enough to let Pilate know that He is king but He is not a threat (Matthew 27:11–12; Mark 15:2–5; Luke 23:3; John 18:33–38; 19:8–11).

Yet Jesus never defends Himself against specific charges, either to the Jewish leaders (Matthew 26:62–63; Mark 14:60–61) or to Pilate (Matthew 27:12–14; Mark 15:3–5). However, He is willing to point out that His accusers are cowards without a case (John 18:20–23).

Jesus' trial before Antipas is consistent with this. Antipas is questioning Him, but they're not having a serious and private conversation. The religious leaders of Jerusalem are right there, slandering Him (Luke 23:10). Jesus does not defend Himself against the charges the Jewish leaders make. He doesn't even open His mouth. He could make an exceptionally good case for His innocence, but He must go to the cross.

Jesus knows Antipas is a "fox" (Luke 13:32), and that interacting with him would be a waste of time.