tight-lipped
adjective uk/ˌtaɪtˈlɪpt/ us/ˌtaɪtˈlɪpt/
口紧的，闭口不谈的
Someone who is tight-lipped is pressing their lips together to avoid showing anger, or is refusing to speak about something.
He's been very tight-lipped about what happened at the meeting. 他对会上发生的事情一直守口如瓶。