The phrase "Let he who is without sin cast the first stone" originates from the Bible, specifically the Gospel of John, chapter 8, verse 7. It is a statement made by Jesus during an incident where a woman caught in adultery was brought before him by the scribes and Pharisees, who were seeking to test him. They referenced the Mosaic law which commanded that such a woman should be stoned to death. Jesus responded by saying that only someone who is without sin themselves should be the first to cast a stone at the woman. This effectively exposed the hypocrisy of the accusers and prevented them from condemning her, highlighting the principle that no one is without sin and therefore no one has the right to pass judgment on others.
In essence, the phrase means that people should not judge others harshly or punish them unless they themselves are completely free of wrongdoing. It implies compassion, self-reflection, and mercy rather than condemnation.