cast the first stone

From the Bible John 8:7, where Jesus tells the accusers of a woman of adultery that "He that is without sin among you, let him first cast a stone at her". All accusers did not do so, and Jesus forgave the woman's sin.

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cast the first stone (third-person singular simple present casts the first stone, present participle casting the first stone, simple past and past participle cast the first stone)

1. (idiomatic) To act self-righteously in accusing another person, believing oneself to be blameless.
   • 2007, March 10, Mr Gingrich, on the impeachment proceedings in 1999. Reported in The Guardian

     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.

• people who live in glass houses shouldn't throw stones