To act self-righteously in accusing another person, believing that one is blameless
I knew I couldn't cast the first stone as I knew I had weaknesses. ... (But) as a leader of the government trying to uphold the rule of law I have no choice but to move forward and say you cannot accept perjury in your highest officials.
To be the first to blame someone, lead accusers against a wrongdoer. "Jesus said that a person who was without sin could cast the first stone.", "Although Ben saw the girl cheating, he did not want to cast the first stone."