clam up
/klæm/
to become silent suddenly, usually because you are embarrassed or nervous, or do not want to talk about a particular subject
(通常指因为尴尬、紧张或不愿谈论某一话题而突然)沉默不语，闭口不言，拒不开口
He just clams up if you ask him about his childhood.
要是你问起他的童年，他就干脆不吭声了。
He's hiding something. He clams up every time we try to ask him about Mary.
他在竭力隐瞒什么东西。每次问他玛丽的事他都支支吾吾。
