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"silent" is also used as a crossword clue. If that's what you saw, the answer is often mum (3 letters).
Definitions
unable to speak because of hereditary deafness
Word origin
From Latin silēns (“silent”), present participle of sileō (“be silent”), from Proto-Indo-European *seyl- (“still, windless, quiet, slow”). Related to Gothic 𐌰𐌽𐌰𐍃𐌹𐌻𐌰𐌽 (anasilan, “to cease, grow still, be silent”), Old English sālnes (“silence”).
Used in a sentence
“How silent is this town!”
“What was formerly performed by fleets and armies, by invasions, sieges, and battles, has been of late accomplished by more silent methods.”
“Ulysses, adds he, was the most eloquent and most silent of men.”
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer2 curated clues
01“Making no sound”6 letters
02“Not making sound”6 letters
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