to shut one's mouth; to hold one's tongue; to keep silent
あの話が出ると皆口を噤む。
When that topic comes up, everyone falls silent.
彼は口を噤んで何も言わなかった。
He kept his mouth shut and said nothing.
秘密を知る者たちは長年口を噤んでいた。
Those who knew the secret had kept silent for years.