To keep one's mouth shut; to remain silent and not speak.
彼は質問されても何も答えず、じっと口をつぐんでいた。
Even when asked a question, he didn't answer anything and kept his mouth shut.
証人は圧力を受けても口をつぐんだ。
The witness kept silent even under pressure.
会議では上司の前で皆、余計なことを言わないように口をつぐむことが多い。
In meetings, people often keep quiet in front of the boss so as not to say unnecessary things.
その秘密を守るために、彼女は最後まで口をつぐんだ。
To keep that secret, she stayed silent to the end.