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Phrase(s): *silent as the dead [and] *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.
McGrawhill's American Idioms And Phrasal Verbs