silent as the grave

Perhaps connected to the idea of taking a secret to the grave, or the idea that dead men tell no tales.

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silent as the grave (not comparable)

1. (idiomatic, simile, of a person) Saying absolutely nothing (especially about a particular subject).

   Synonym: silent as the tomb

   The investigation is ongoing, but all the suspects have been silent as the grave so far.

   • 1881–1882, Robert Louis Stevenson, Treasure Island, London; Paris: Cassell & Company, published 14 November 1883, →OCLC:

     “Livesey,” returned the squire, “you are always in the right of it. I’ll be as silent as the grave.”

   • 1884 December 10, Mark Twain [pseudonym; Samuel Langhorne Clemens], chapter XL, in The Adventures of Huckleberry Finn: (Tom Sawyer’s Comrade) […], London: Chatto & Windus, […], →OCLC:

     “Well, then, if you’re bound to go, I’ll tell you the way to do when you get to the village. Shut the door and blindfold the doctor tight and fast, and make him swear to be silent as the grave, and put a purse full of gold in his hand, […]

2. (idiomatic, simile, of a place) Deathly still and silent.

   Synonym: silent as the tomb

   The abandoned village is silent as the grave.

Note: these are translations of a verb construction, to be as silent as the grave, rather than of the adjective.