What does Luke 22:57 mean?

Peter is beginning to realize that he may have made a mistake. He has followed the crowd of priests, guards, and servants who arrested Jesus to the home of the high priest. He has joined some of the bystanders at a fire. But he doesn't want to be identified. Unfortunately, a servant girl does recognize him. She tells the men at the fire, "This man also was with him" (Luke 22:56). Peter denies it.

When Judas helped the priests, Pharisees, scribes, and elders arrest Jesus, he didn't fully understand what was happening. He apparently didn't know they wanted to arrest Him so they could have Him killed. Peter is in a similar frame of mind. Jesus warned that Peter would deny Him three times that night. Peter insisted his loyalty would never fail, not even if he were sent to prison or killed (Luke 22:33–34). Peter doesn't seem to realize that Jesus' warning is coming true at this moment.

We don't know why Peter is here in the first place. Is he trying to prove Jesus wrong? Does he think he can rescue Jesus? Whatever he thought initially, he now realizes he is in the courtyard of the high priest. Within earshot, members of the Sanhedrin—the ruling Jewish council—are trying to produce charges against Jesus. Peter is sitting amongst some of the men who arrested Jesus. He's in a very precarious position. Not only can he not save Jesus, but it doesn't look like Jesus can save him.

So, when the servant girl accuses him of being one of Jesus' disciples, all he can think to do is lie.