clam up

To clam up means you refuse to talk or you are unwilling to respond to someone who is asking you question. In the literal sense, a clam closing would be unable to make a sound should it be able to when the clam is open, so when thinking about it figuratively and as a cliche, it means to remain quiet with your mouth closed. To clam up is often used as a way to describe someone suddenly becoming quiet and refusing to answer a question.