Why in Acts 23 does Paul call the high priest a whitewashed wall?

Answer 1

The White Washed Wall refered to is that of a tomb, or grave that has been decorated to look nice on the outside, but is filled with dead men's bones. See Matthew 23:27. Contact with the dead would defile the High Priest and prohibit him from officiating at the trial.

Striking a prisoner before conviction was against Jewish law and Paul is pointing out the hypocracy of the High Priest who was willing to break the law in an effort to intimidate Paul, thereby corrupting the judicial process.