“When they were little, John and his cousin were thick as thieves.” 「小さいころはジョンと従弟は泥棒みたいに仲が良かった。」


When two people are “thick as thieves”, they have a close relationship and keep each other’s secrets. They aren’t really thieves, although they might get into some mischief together. But they cooperate and work together just like partners in crime who are loyal to each other.