silent as the dead
*silent as the deadand *silent as the grave
completely silent. (Has ominous connotations because of the reference to death. Usually used to promise someone that you will be silent and therefore not betray a secret. *Also: as ~.) I knew something was wrong as soon as I entered the classroom; everyone was silent as the dead. Jessica is as silent as the grave on the subject of her first marriage. If you tell me what Katy said about me, I promise to be as silent as the grave.