"Mum's The Word"
Context: The use of "mum" to signify silence is widespread. Starting as an inarticulate sound, as we say "mumble," the word then became "momme" in Middle English, and meant "soundless." A "mummer" is an actor in a dumb show. In Merry Wives of Windsor (Act V, sc. ii, l. 6) Slender tells Mr. Page how he recognizes his daughter: "I come to her in white and cry 'Mum.'" Sancho, suspecting that it was the Duke's servant who impersonated the Countess in the Duke's practical joke on Don Quixote, tells his master:
. . . Adad, sir, . . . you may think I'm in jest, but I heard him open just now, and I thought the very voice of Madam Trifaldi sounded in my ears. But mum's the word. I say nothing. . . .